Hi Guys,
I have no idea about milling, but i'm assuming so correct me if i'm wrong, Could it be the tool used, maybe it is not straight enough or the diameter setting is not exact? if the Mach thinks that the tip diameter is lets say 6mm but it is actually 6.02 then i would imagine that you'll get an error just as much on all axis. and the same if the tool tip vibrates at that rate.