Windows only it seems like.
1. Create a document in Photoshop, 100×100px, 72ppi
2. Copy the contents
3. Create a web/mobile-based document in Illustrator
4. Paste the copied image

Illustrator pastes 99.9875×99.9875px image.

It turns out Photoshop uses pixels-per-meter (biXPelsPerMeter and biYPelsPerMeter) to define the reoslution.
72ppi is exactly 2834.6472 pixels per meter.
Unfortunately, the value must be an integer (thanks to BITMAPINFOHEADER), so the convention is to use 2835 (which is 0x0B13 in hexadecimal).

But Illustrator is going by the letter of the law (not the spirit) and converting 2835 to 72.009 pixels per inch.

2835 / 2834.6472 × 72 = 72.0089611152

This is lame. If it sees incoming data spec'd as 2835 pixels/m, it should "know" that this is 72ppi.

What integration and collaboration can we talk about, when two main graphic whales of Adobe can’t find a common language to exchange data? What a shame.

Thanks to John Goodman for initiating this investigation and helping with it.

This is very similar to the infamous 'extra-pixel bug', where Illustrator KNOWS the intended size users want to export, but IGNORES it, adding an extra pixel, randomly, because it nails the coordinates to its cursed internal grid — https://illustrator.uservoice.com/forums/601447-illustrator-desktop-bugs/suggestions/37694659-extra-pixel-added-to-image-size-when-exported