The ways vendors measure laptop battery life and the way you are using it are likely vastly different. If you allowed the system to operate in a max savings mode, turned off your wireless, dimmed your display, etc, you would increase battery life.
If you are watching movies, running background apps, cruising the net via wi-fi you will have less time.
If you are getting 2.5 hrs while doing these sorts of things, you are probably doing well and shouldn't worry about it. Good luck!