Expanded shapes get an additional redundant anchor point
1. Draw a square with a stroke set to the inside or outside (doesn't matter which as long as the stroke isn't centred)
2. Expand the square and instead of the 8 anchor points you would expect the compound shape to have, the expanded square has 10 anchor points.
The top right corners have 2 anchor points each, created directly on top of each other.
This happens with ANY shape with corners and a stroke that isn't centred.
You can't see the extra anchor point directly because it is in exactly the same location as the correct anchor point.
To prove there is an extra anchor point, switch to the Direct Selection tool and you'll see that all the corners except the top right corners have a Corner Widget. The top right has no corner widgets because technically there is no corner - there is no path that changes direction because the anchor points are directly on top of each other.
You can also prove there are 2 anchor points instead of 1 by using the Astute Graphics tools. When the corner is selected the Astute info panel shows 2 anchor points selected when it looks on screen like there is just 1 anchor point selected. You can the use Astute tools to remove the extra anchor point and Voila the corner widgets reappear and the shape now has its correct number of anchor points!
Egor Chistyakov commented
There is an option in Pathfinder palette — Remove redundant points. It helps a bit.
But AI keeps switching it off from time to time (as it also does for snapping for Tabs).
Yo can vote for fixing this here: