First time on a laptop and confused by the MacBook Pro 10 hour battery claim. I've read some reviews stating that they're not getting close to the 10 hour mark but I'm really falling well short of it. I feel like at best I may get 5 hours, maybe 4 on my MacBook primarily using iTunes and Safari. Three hours ago I closed the screen with the power still on at 31%. Opening it back up and waking the screen I'm now at 23%. Is this normal?