I'm not an electrician or master grower by ANY means, but I think I have to agree with Ocotillo. If opening it won't void the warrantee, you should do it. I can see 1 of 3 things leading up to this:

1) A mislabeled ballast.(Only a sticker right?)
2) As was said earlier, maybe it only runs 150w. Highly unlikely if it is labeled 250w.
3) Someone shipped you the wrong light and you got 100w for free.