iPhone 4: Usage and standby time same, reboot fixes it

I am facing this weird issue with my iPhone 4 which is jailbroken on iOS 5.1.1. Although I have to admit I never noticed this before so do not know from when this is occurring.
This is what happens..

1. I charge my iPhone 4 till 100% and then remove the plug. This also resets the standby and usage time.
2. I check after few hours and see that both the times are same.
3. I reboot the phone and when I check after few hours(phone idle or very very less used), the usage time is just a bit changed but the standby time is more which is an ideal scenario.
4. I keep phone for recharging and charge till 100% and the cycle starts from step 1.

I am unable to understand what the problem here is and how come a reboot always fixes this.

So how do i exactly find what is running behind and eating battery. I was of the opinion that if i just press the home button the app goes into background for sometime and then eventually is killed and even if it still shows in the task manager (double click the home button), it is sort of inactive right ?