My extremely short hypothesis.

[PrincessZombie]

we don't know that this WAS the first time cordy took out a vamp. she may have staked a vampire offscreen, we just didn't see it. and she had been hanging with the scoobies for two years at the time of 'graduation day'.
whereas, amanda fought and staked that vamp after finding out vamps were really less than an hour before, without any traing whatsoever.

I think everyone would have noticed if Amanda had Slayer powers. Someone would have noticed her superhuman strength. It's kind of noticable. It's not that hard to take out a vamp, you just have to stake it! And she wouldn't be a potential if she was already a slayer.

 

[Lyri]

ok, that's a fair point.
BUT my comment about amanda was supposed to be a joke, but everyone took it seriously!
it backfired on me!!