The grid unit is 1 pixel.
Below is two overlapped rectangles:
Underlying one has blue fill color an black outline color. Also it has
Align Stroke to Inside setting:
As you see, it also has stroke weight set to 1px.
Nevertheless, the overlapping rectangle, which has no outline and blue fill is obviously slightly bigger.
Also, according to the grid, it is evident, the outline of the underlying rectangle is thicker than 1 px.
How can I fix this?