Probably contract volume. Verizon full retail of the S2 was the same but with a 2 year contract they knocked off $50 and you couldn't cancel that line for 2 years. AT&T was the same way i believe.
Hey jeffahales! That's a great question. It seems likely that it's volume, as mentioned, but I'm going to ask up and see if we can possibly get a more specific answer for you. I'll update you as soon as possible!