By my understanding, the explanation is this:
- the Entry is right-aligned within its cell
- but then the Grid is set to align fill and expand, so it expands empty space beyond the Entry and the cell it's in
Assuming I understand this guide right, then it does kinda make sense to me.
https://developer.gnome.org/
gtk3/stable/ch30s02.html I found that hexpanding the Entry too got around this, but it may not be exactly what you want.