The key is that both were randomly assigned to users - you’d never know if you’d open a thread and be a moderator. If you posted in the thread you couldn’t moderate.

And about the same frequency you’d be assigned to metamoderate, basically being asked if a moderator’s “vote” was a good one or not (you didn’t have to fully agree you’d do the same, just that it wasn’t bad).

Someone who scored low in meta moderation would get less or no moderator chances.