I dunno about
@cheque_raise 's reasons, but from my perspective - all the milled chips I've seen in pictures would bug the ever-living snot out of me if I owned them (or so I speculate - I haven't actually seen any in person). It seems to me like the milled recess is an obvious artifact, an alteration in the chip's "natural" form as produced by the original compression mold, distinct from the very slight recess produced by the mold within the molded circle. The artificial recess is deep, has a smaller diameter than the original molded interior circle / recess boundary, and is usually off-center (by necessity) both with the mold and with the chip edge.
If I ever want to relabel hotstamps, I suspect I'd want to try sanding down the hotstamp rather than milling it out. That sounds like a crazy amount of work, though, so I probably won't ever do that.