Offset path has a math error at 4 decimal places

Question

I have Illustrator CC 2014 running in Mac OS X 10.9.3, and discovered tonight while making a test pattern that if I do an offset path of -0.0005" it doesn't calculate the size of the resulting objects correctly. If I do +0.0005" it works fine, if I do -0.0015" it works fine. I obviously haven't tested exhaustively, but there is definitely a reproducible error there. It's not even off consistently in the width and height. The sizes listed refer to the size of the individual squares in the 4x4 grid of filled rectangles.

Answer 1

The black squares are what I'm building. So I didn't have to go one by one and apply offset path, I did three across and then noted that the next row above would be .003 difference, so I could apply a .0015 offset to all items in the row and move it up, then repeat. This works well for a while, but then it starts to get off. It doesn't appear to be cumulative which was my fear. The black squares I checked each one and redid them if they didn't come out right. The blue ones I just did an offset from the previous row. At .015 it should be .065x.065 and they are .065x.0649. That's the behavior I normally see, not a big deal. In that group, all 16 squares were the same, but in the .018 group above it, the first two rows of 4 were .062x.0619 while the 3rd row of 4 were all .062x.062 and then the top row were off again. By the top, the ones that are off are still only off by .0001 when they are off. So that implies that this is at least a rendering thing, it thinks the squares are what they are supposed to be when it applies future offsets.
I'm not sure if my original example is the same though. That was off by .001 in one direction and .0005 in the other which is 5-10X more than these. These are the issue I've always seen that at 1/10,000th of an inch are tiny differences and not really important for what we do. The one I started this thread about is much bigger and a definite problem. I'll post my latest test on that separately.