I think admin can demote any rank why you say Here and Legendary is semi-permanent? Same applies to each and every rank I guess.
He's talking about a different a different matter
, there is a bug where demotion for Newbies, Hero member and Legendary members is not handled properly by the forum. See examples where accounts with less than 400 posts still have hero member status. Sure admins have the power to delete most of the posts and demote a rank or two, but I haven't heard of such cases. Anyway, this is quite OT
OP, if by some chance you had a higher activity than you currently have the most likely cause is that you:
deleted a bunch of old posts/had some old posts deleted by mods recently
,the ones that were the only posts in a certain activity period