I think it's a combination of things.
You can't measure a camera's battery life purely in hours or number of shots. It's a mixture of the two, as well as how long your LCD screen has been on and at what brightness, etc.
Also, there is no way to reliably check a battery's capacity, only it's voltage. The two are usually related, but there are other factors like heat that can affect the capacity estimate. That's why Canon only gives you a few "ticks" on the external battery meter. If you saw it go from 100% to 89% right away, it would concern you. But seeing it go from 5 ticks to 4 wouldn't as much..
Or, you could have a faulty battery.. What does the "recharge performance" say for that battery?