Hi @spraguey,
There was once a thread where we collectively obsessed about this effect, which you may find interesting if this has been bugging you for 2 years 
Bottom line: assuming everything else is 100% correct (e.g. programmed vbit angle is EXACTLY the angle of the actual vbit, don’t trust the manufacturer that it is any given value with high precision), this is probably down to tool deflection when cutting the straight sides (the corners are where they should be), and usually just running the vcarve a second time will address this (just because there is very little material left to (re)cut, so the tool/router basically sees a very small load)
The OTHER usual suspect, that you have not mentioned, is incorrect Z zero setting. And this is particularly true if using a vbit that has a small flat on the tip (they all do, to some extent). The Z zero should be raised by a value corresponding to that flat tip width and the vbit angle (we have a calculator somewhere for this…) such that the virtual point of the vbit is a stock surface, if that makes sense.