I had a biophysics professor in college who was very fond of saying that any time infinity comes up in physics, it was natures way of telling you that you screwed up the math somewhere.
Difficult to answer because there’s more variables than you mention here (spring bypasses?, length/thickness of led wires?, low or high drain batteries?, old produce XP-G2 or new produce XP-G2?)
Personally, for every build again I look at the led test data, the battery test data, and at the components that I will be using, and then I do the calculation.
To be “protected”, you need the board on one end, usually the negative, and the sense-strip running to the positive end. To secure that strip at the positive end, you need a cap on top of the nekkid positive terminal, and usually that’s with a button-top cap.
Most are double-wrapped, with the sense-strip running between the 2 wraps, others like the panny-B just have Kapton tape covering the strip, which is then visible.
Most probably figure that if you’re sticking a cap on it already, you might as well make it a button-top so it’ll look like conventional cells and fit in holders, etc., without the need for nipples, springs, etc.
18650 batteries are made as flat tops because they are almost exclusively used in battery packs so a tab must easily be welded onto it.
Protected 18650’s are for a niche market that the producer never intended, protection therefore is always added as an aftermarket process by a different manufacturer.
I think button tops are added then because they are more versatile, and there are not many devices that exclusively work on flat tops.
Tennergy made what could be considered protected flat tops ...
My guess on why 3.7 volts is because nimh and nicad etc are 1.2v and three cells combined equal 3,6v which if I remember correctly was a number more commonly used than 3.7v. I'm sure there is a better answer ..but hey "there are no stupid answers "