I have a question!

So I have an area 7.6 in. by 9 in. and apply a rectangular grid with 45 horizontal dividers and 38 vertical dividers with a .5pt stroke, basically creating a grid with .2 in. squares. I then applied a separate rectangular grid that has 90 horizontal dividers and 76 vertical dividers and with a .5pt stroke, creating a grid of .1 in. squares. The idea here is that I can offer a dungeon map with either a 10-foot square grid or a 5-foot square grid.

If I have both of those rectangular grids visible, they do not line up as they should except at the outermost edges. Each of the .2 in. squares should "contain" four of the .1 in. squares, but it's not lining up that way, except on the extreme edges, as a I mentioned. This is bothersome, to say the least, since I would have to adjust or redo things since the grids don't line up as they should.

Any idea why this happens?