I think it just might be wear and tear on the parts making up the throttle advance. The parts involved with the throttle advance were never made with the tolerance and precision of say, lower unit gears. Plus, there’s play involved all the way through.
While turning the throttle handle if you look closely, there are at least 4 or 5 points where there is play…the first set of gears under the handle (it doesn’t matter if they’re stamped or cast, but the earlier stamped ones seemed worse with more play), then there’s play at the pinion gear and throttle gear, then the pin in the vertical throttle lever moves some, and finally the upper part of the throttle lever itself can move where it’s secured in the bearing clamps.
All those little, consecutive incidents of play in that progression add up to being able to turn the throttle handle a bit before it has any effect on advancing the link and stop arm attached to armature plate. And that translates to inaccuracy in the tightly grouped readings on the indicator.
That’s just my humble opinion.
