How'd I get that postbit thing all of a sudden?

When you hit 100%, and stay there for a certain time, then you've got it permanently, as long as you're over 75% activity (or something around that mark).

Yeah, but he's at like 79%

I'm guessing it is given to members over a certain longevity?

I believe he changed it to how I had suggested. Hit 100% once and then stay above 75%. So I'm guessing, if you were ever at 100%(which is true for Thanatos,) if you stay above 75% you get to keep it. You never stayed at 100% for a month, so that seems to be the likely case.