Just speculating. Don't hit me, please.
In another thread today a member had some problems with an app making downloads, because they were not working in the background. He got around it by leaving the app open, but using the power button to put it to sleep.
It made me wonder if say, leaving Safari open with a page that does updates (like advertising banners) would continue to use extra battery even if the iPad screen was off. Many apps continue to use the internet to update as long as they are active. The question is, do they remain active if on, even if the iPad is in sleep mode?
I admit it's far fetched, but it could account for some of the longer charge times or overnight battery drains I've seen mentioned in this and other threads.