Probably because the compliance changes. The farther you go from the origin, the easier it is to push the tool away from its intended cutting path. A gantry-st

Probably because the compliance changes. The farther you go from the origin, the easier it is to push the tool away from its intended cutting path. A gantry-style machine, on the other hand, has the same rigidity anywhere in the X-Y plane. Rigidity is a bit less as Z changes (due to a longer moment arm), but it typically doesn't travel as far.