This all comes from Zen, so if there are any errors in fact it's on him. But I used to follow the man pretty closely (not, not stalkerish, just interested

) and he seems to be the type who always wants to know WHY and then does the research to find out.
Back when the 510 was created, most everything was a cigalike. The only box mods to speak of were made from pre-existing electronic boxes modified to be usable - and they were being made by individuals in their basement shops or garages. Throw in the fact that also at that time, there wasn't much of a hobby aspect to vaping as it was all about quitting cigarettes.
So most of the market was cigarette-sized and shaped gear. It's a form that smokers were accustomed to and were comfortable with.
The dimensions of the 510 were set because that's what would fit on something the size of a cigarette. No one with electrical or engineering knowledge designed the thing. And back then, when we were all only using 5 watts or less of power it worked OK.
The next part is hazy since I don't remember the exact numbers. But when people DID do the math for what was needed to efficiently transfer 20 or 30 watts they found there just wasn't enough connection conductivity to do that at a high level of efficiency.
And thus the P3. Pro Vape's engineers understood the issue but were put into an unwinnable situation. Tooling, production costs, and inventory for sellling two variations of the same mods would be expensive. And they couldn't just move to P3 exclusively since the atomizer makers would have to follow along. Even though Pro Vape offered the plans for free to anyone, few companies wanted to change their production which was already making millions of pieces.
So they tried to make a mod easily convertable between the standard 510 and the electrically better/physically stronger P3. It was a clever solution with the adaptor nut and lock ring, but as DPL pointed out they didn't get the 510 post exactly right.
And this reply was probably at least 5 times longer than it needed to be, but hey, it's a Sunday. What else do you have to do on a day of rest?