Height and length are absolutely comparable. They’re the same thing. The distance from one point to another in a one dimensional straight line.
I agree with you it’s a bizarre scenario for curved guys and measuring both topside and underside would in theory work, but it would just be too difficult to get any sort of standard on the underside with various possible starting points and the balls in the way, etc.
I was actually somewhat wrong about a topside measurement “shortchanging” and if you read what sdp said a few posts back, a topside tape measurement would actually be legit for upward curved guys but the tape would have to be completely taught with no slack or draping over the head.
And not to be a dick, as I think we’re on the same general page, but the driving/road comparison also makes no sense and is irrelevant in this discussion.