Skillful - retired achievement stuck at arbitrary number

[Prophet of Flames.2783]

So I'm trying to figure this one out and have no idea where to search next, wiki gives me nothing, google says nope, so maybe someone here knows.This achievement is retired, has been since before I started playing the game, so I'd get it if I couldn't complete it anymore (even though that's kind of random to do to hurt peoples ability to get AP like other players) but that's not actually completely the case. The achievement is at 609 out of 1000 and I have no idea how or why, or how to get to 1000. If I couldn't do it at all, it would've just stayed at 0, right?

What does this achievement track now, and why does that go to 609 but no further? It seems like such a random arbitrary number.

[Khisanth.2948]

There was probably a bug after the change that still continued to track something.

On my alt account it is at 480 despite the fact that the oldest character on that account being created several months after the change that should have disabled the achievement.