Apple got a lot of bad press a few years ago for massively overestimating their battery life and is now quite a bit more conservative. They've gone from claiming 6 hours to claiming 8, but at the same time they've shipped lower power CPUs and doubled the size of the battery. There was a Kickstarter for an open source compatible laptop with very similar specs to the MBP floating around last week: they were also claiming 8 hours on battery, but they were shipping a battery half the size of the MBP. I guess they think Linux users keep the screen turned off.
Adjusting the brightness has a big impact on battery life for the MBP. Cutting it to 50% can give you another hour or two. I have gfxCardStatus installed and so disable the nVidia card if I'm going to be using it on battery for a while.