If you place a ruler(line), rotate it 45° and zoom in, the line is not a straight line. You can see a stair effect.

Moving over it with the cursor, makes artefacts on repaint.

A 45° line from top-left to bottom-right should be always drawn as 1px right and 1px down.

I think the math-lib in the background of illustrator is written with a very very poor quality, so that this in trues is a rounding problem from float to integer.