Samsung Galaxy S III (T-Mobile)
T-Mobile and AT&T are showing 8 hours. Verizon is showing 15 hours. Sprint and US Cellular are showing 9.2 hours.
Some seriously oddball battery times.
However, I think most of what you're seeing is simply different ways of measuring battery life. Unfortunately, the industry has yet to adopt a standard for measuring such things. So each company does it a little differently, and thus the numbers vary quite a bit.