This one is about 0.03 units in length. That's not so small to be because of a rounding or limited precision error... Probably it's just from sketching the cutting rectangle more free-hand style instead of locking on with a snap.

In many situations you can do a free-hand sketch (without snapping, I mean), but when things come very close to one another, that's when it is better to lock on with snaps to ensure you get exact clean results.


But, if you use your rotation method to move the seam edge out of the way, then this small offset is not a problem since there isn't any other edge nearby there.

- Michael