When I test this, I see the scale is calculated based on the resolution of the image...
If I resize an image on canvas, the effective resolution changes and the scale value in the panel changes as well.

...but there is a difference how different formats are handled! And it’s a mess.
Technically, an image can have no resolution value specified.
In this case Ai assumes one to be 72 ppi.

But if there is one...
Ai respects this value for TIFFs, PSDs, etc. — but ignores it for JPEGs! The scale for these is calculated as if it’s 72 ppi, no matter what is written in the file...

When I check this in InDesign, JPEGs are handled fine (and InD shows the file’s own resolution and the effective one (as scaled on canvas) as separate values).
This problem was reported here at UV previously.

But your screenshots show TIFFs however...
And one is 3149×2158 px, while the other is just 644×441 px... And the file size in Bytes differ, and the resolution value: 125 ppi VS 200... as if these were different files. I think they are :)

So I recreated two TIFF files with these parameters.
What I found is that the scale is calculated properly for TIFFs ONLY if it’s layered! If the TIFF is flattened (that is when you open it in Photoshop, it shows 'Background' and not 'Layer 1') — the resolution is again assumed to be 72 ppi and the scale is calculated , even though it’s placed on canvas respecting the resolution.
This is not good at all :(
Definitely needs studying...

Very randomly, the scales for placed links in Illustrator 2025 is not at all accurate. Rarely now it says 100%, but when I resize images in Photoshop and place the them as links into Illustrator, the scales read 36% or 57.6%, and now today I got something that says 48%. It should say 100% and this is a huge problem when you have multiple people working on these files for production. It's important that these images are scaled at 100% when sending to the printer. This is a major blunder on Adobe's part. I don't understand why this would get so buggered. I know this is an Illustrator 2025 issue because when I open these files in Illustrator 2024, the scales say 100%.
Please fix this ASAP!

ご返答いただきありがとうございます。
メールをお送りしましたのでそちらよりデータの送付をお願いいたします。
ご協力ありがとうございます。

MIE様
すみません。
「同一ファイル内の画像で、何点かは正常な％で表示されている」は、
仕事のデータのため「こちらでの共有が難しく」、
メールを頂ければ、ダウンロード先を返信いたします。

サンプルで、新規に作成した TIFF を、配置（リンク）しても、
2024は 100％ で、2025は 20.571％ になったので、不要かと思いました。
サンプルファイルを、コメント欄の「ファイルを添付」で、
アップロードしたのですが、それは、共有ではなかったんですね。

350dpi の画像を配置（リンク）した場合、を調べ直してみると、
2023（27.9.1）までは JPEG と TIFF ともに 100％
2024（28.7.5）は JPEG が 20.571％　TIFF が 100％
2025（29.3.1）は JPEG と TIFF ともに 20.571％
になるようです。

hide様
重ねてのお伺いで失礼致します。問題を確認できるデータの共有はしていただくことは可能でしょうか？同一ファイル内の画像で、何点かは正常な％で表示されているとのことですので検証用ファイルとしてご共有してくださいますと大変助かります。（ご共有していただいたファイルは製品の向上のため検証データ用として慎重に取り扱わせていただきます。）
こちらでの共有が難しい場合は直接メールをお送りさせていただきますのでご返信していただくかたちでファイルのご提供をお願いいたします。

29.3.1 で、TIFFだけ、JPEG と同じようになったのかな？
（PSD と EPS と）TIFF は、解像度ではなく、寸法で％を表示していたから、
大きい画像を、小さくリサイズする場合に、参考にしていたのだが、
このまま参考にすると、五分の一の粗画像になってしまう。
さらに、同一ファイル内の画像で、何点かは正常な％で表示されている。

Seems related: http://illustrator.uservoice.com/forums/601447/suggestions/49596260

Seems related: http://illustrator.uservoice.com/forums/601447/suggestions/48855905

ご報告ありがとうございます。
「拡大・縮小」の％が誤って表示される該当のTIFデータのご共有をしていただくことは可能でしょうか？よろしくお願いいたします。

macOS 12.7.6 intel
Illustrator ver.29.3.1

リンクパレット「拡大・縮小」の％が、TIFF画像の場合、正常に表示されない。
TIFF画像を配置しただけの100％の画像が、20.571％と表示される。
Illustrator ver.29.2.1 までは、起きていなかった。

Illustrator (2025) v29.3.1

I have a similar issue. For some images linked into my .ai file, they are placed in at 100% scale, however in the Links module, they are registering at 36% (or something close to 36%). It's not all images doing that. In fact, I'll open the same file in Illustrator 2024, and it will register 100% scale in the Links module. This is a bug in AI 2025. Please fix!!

Illustrator is correct by saying the image now has 300ppi — this is the 'effective resolution' of it, the 'real pixel density' this particular piece of raster now has, and it matches the original resolution the image had when you placed in on canvas. If you scale it further — you’d get it grow larger, because the pixel density increases.

As for the PSDs which stay scaled differently — I’d like to see that, if possible, when it happens for you. It should work the same way, but who knows?

Thanks Egor, that makes sense, but it's not adding up to me... The links panel shouldn't say the image has 300 ppi if it's actually assuming the image has 72 ppi. Also, there are some .psd's that I embed that remain scaled at 100% instead of going down to 24%. Do you know why this happens with some .psd's and not others?

This is actually an intended behaviour, although not as evident as it might look.

See, Illustrator has an internal 'base' resolution for raster images, which is historically is 72ppi. When you embed an image, Ai no longer knows about its original resolution (because this is something what a linked file can have, not an embedded piece of a raster), but have to display it anyway, scaled to the size you used. So it assumes it has 72 and rescales it.

If a linked file does not have a resolution values stored in it at all (there is no punishment for it), Ai uses 72ppi also — as many other applications do. So when you put an image which is 120×120px into a document with 'pixels' units in Illustrator, it gets put as 120×120 exactly because 'undefined 72' matches with Ai's 'internal 72'.

72 / 300 equals 0.24 exactly — here you have your 24% of the scale.
Does it make sense for you? Please reply back.

1. Windows 11 Pro
2. Illustrator v 26.5
3. When I place a linked image at 100% scale, the links panel shows it scaled at 100% which I would expect. When I go to embed that same image, the links panel shows that it's scaled at 24%. The image itself doesn't visually shrink, but the links panel changes the information for some reason.
4. I expect the links panel to show 100% scale when the image gets embedded. I don't understand how it's scaled at 100% when linked, but 24% when embedded, when all of the file information is the same as when linked.