Grid tool creates inaccurately sized grid

Hey everyone, I have been trying to make a mathematical drawing grid in illustrator, but I'm having issues with the grid tool. To simplify my problem I've simulated it with simple numbers.

I have a 300x300mm artboard which I want to fill with two grids: one with 10x10 squares, and the other with 5x5 squares.

So in the grid tool I entered these values:

You'd expect the above mentioned grids to appear perfectly, but they did not. Instead, the two grids together looked like this:

When I make an actual 10x10 square and compare it to the big grid, it looks like this (the 5x5 one gives a similar result):

Does anybody have any idea why this is happening and how I might be able to fix it?

Thanks in advance!