Drumming and generally keeping rhythm is a physical thing. You move your body parts (e.g. bopping head, swaying hips etc.) and it couples with how and when you're playing.

Are you suggesting his movements in this one measure were so unique that they exactly doubled the length of one eighth note?

At some point I think we have to put the “it was a freak accident that was repeated with precision over and over, forever” argument to bed.

With n=1, for me there is a groove or three that are just muscle memory and I'd need a longer while to sit down and think if asked write them down. They came out from noodling around and coming to "huh, that sounds cool" and I imagine that's not something particularly uncommon for other people too.

I think we lost the thread here. I wasn't suggesting Lars had millisecond-level intrinsic timing and an innovative approach to music theory. I was rejecting the idea that this beat came from incompetence. I think developing an interesting groove and being able to play it consistently in a song, and in live performances for years is evidence of drumming competence.

You don’t play music, do you? It’s absolutely possible to learn a rhythm, groove or melody wrong, and to rehearse it until it sounds so right, the anything else sounds wrong to you. But your audience would notice.

As a musician, I do not find this surprising at all. It's easy to internalize "incorrect" rhythms and pitches.

"practice makes permanent"

That's why you want to practice _slooowly_ but perfectly before you speed up. That way mistakes can't take root.

Did he do that consistently or is it just an artifact seen in a single very heavily edited record? Is the same timing seen in this section of live concert recordings during those years?

I have not been a metal head for many years but my recollection is that the live albums and bootlegs have timing that varies all over the place. Especially on MoP and Battery, and a few other notable ones (The Four Horsemen) which have complicated rhythms.

Yes, that's how playing music works. Whatever you do on your first playthrough is what you'll do on every playthrough if you don't specifically try to change it. Consistently reproducing a mistake is generally much easier than unlearning the mistake.