[Brian]

One possible explanation for stuff like this - in order to make your back button faster, we keep some number (5? 10?) of pages in memory, even if you're not looking at them right now. This avoids needing to fetch everything again from the network when you hit the back button.

This would include pages that use Flash. So if you just looked at a bunch of YouTube videos in a row, several of them are still kicking around in memory, in case you hit the back button and want to watch them again...

I'm not going to claim that this is the entire explanation of what folks are seeing, but it is likely a contributing factor.