Has to do with the system being based on "pixels", for lack of a better term. In other words the lines we draw may not fall on the "pixel" boundaries all nice and neat like. So discrepancies show up.
A thing to remember, Aspire is not sold as a solid/mechanical modeler, it is designed for artistic things so a few thousandths of an inch do not matter. But of course, we all decide to do mechanical things that need to be spot on so then the trouble starts.
When I do real mechanical stuff that needs to be spot on I turn to a couple of different programs, Fusion is one and I also use something called Hexagon which is no longer around. Fusion works extremely well and is free unless you are a certain size business. It also has a serious learning curve.
So, whatchya making?
Low Profile CNC Router Vise