Toshiba nb250 battery power?

I bought a TOSHIBA mini net-book a couple days ago from argos. It all seems fine, but i'm pretty much of a newbie to this. My question is, Why does it only last 3 hours when it's fully charged & on 100%. When it says it should last up to 8 hours. Is something wrong with it? It'll take allot of stress out of my head once i find a legit answer. Thanks guys.