256 Minutes or 277 Minutes?

I just purchased a 64 gig P2, E series card today. I have a training seminar that I'm shooting coming up and it has some pretty long sessions so I'll be recording in DVC Pro on my HVX200a. EVERYTHING that I have seen online says that I can expect to record 256 minutes on this card. I just plopped it in the camera to check it out and record a test and it says 277 minutes. It's the same whether I'm shooting 30p, 60i or 24p...so do I really get 20 extra minutes? Is there something I'm missing?

I appreciate your feedback. Thanks!

Scott-

Scott Whitney
Whitney Media Productions
http://www.whitneymedia.tv

Hmmm interesting Steve. I wonder why the 277 is coming up. Anyway, I'll have enough cards to cover myself...I wanted to see if I'd be able to fit the entire day in 2. Thanks.

Scott-

Scott Whitney
Whitney Media Productions
http://www.whitneymedia.tv