As others have stated, this information doesn't seem useful as there is no indication of how these measurements are obtained, and the article itself says it is an average so not particularly useful.

More relevant to UA, I'm under the impression that the extenders simply are stored on the aircraft? The "must pre-reserve" response that they got is a bit surprising to me.

I've definitely noticed a variety of seat belt lengths. In particular there was one regional aircraft type where they were really short. I want to say it was on CommutAir's Dash 8 200s but obviously those aren't around anymore.