I was checking my profile, and it appears that within the space of one or two days where I've not been very active at all, the little rankings (xyz Contributions (#abc)) seem to have placed myself, and some other members' profiles I looked at, way up the list. I had been sitting down in the #800-#1000 range from memory. It's as if other members with equal or better stats than me have disappeared or are being omitted from results? As I said, I've not been very active over this time.
I'm not complaining, just wondering what's going on.

I think it's just a matter of you having fairly low counts and that you've had quite a bit of activity in the past week or so (lots of posts, and several "post comments" with reputation points associated). This can make your rank jump quite a lot. It's all relative. At your ranking levels, a reputation increase of 15 points can make you jump nearly 100 rank positions. And 10 additional posts can make that rank position jump by about 50 position. I think you just didn't notice it, because it jumped quickly.

I don't think that anything changed about the ranking system or points awarded.

Sorry, you looked at it while it was broken. I was working on having it only put members who were active over the past 90 days in the top list but it didn't work so I reverted it.