Well. Investigation with DMM. Varying resistance across about 230 degrees rotation (eyeballed it before, got it wrong). Both ends do have large conductive ranges. Gap in center between the two resistive track contact is where the wiper trace goes, so rotating the servo to 270, or -90, would leave the wiper wired to...itself. So there's some range beyond 0 and beyond 180, but not a lot. I suspect (not going to ring it out), the 0-180 range likely aligns with a linear range similar to that you're describing, maybe 1/6 to 5/6 of a regulated source, and forcing the servo beyond those points may result in 'unspecified behavior'.
Far enough down this rabbit hole, not going any further, but it was interesting.
One chip "PM622W", other "KC9102 2008A"