Comment 4 for bug 1342911

all of the boolean path operations depend upon the idea that the original object will be "snapped" onto a grid before performing the boolean operation. In the case of the x coordinate this may not be a problem, if the original x origin was an integer. In the case of the y coordinate this normally will be a problem because the y coordinates get inverted during the calculation, and if the page height is not a perfect integer then the inversion will lead to round-off error.
    The latest reference I could find on this is:
http://thread.gmane.org/gmane.comp.graphics.inkscape.devel/40786
   and bzr rev 12420.
    The original size of the grid was 1/32 pixel = 0.03125
    The current size of the grid, in trunk, is 1/512 pixel = 0.00195
    These numbers look to be consistent with the above comments.