They may be better memory management going on in Safari too. I keep lots of windows open, and apps, and they all have graphics. I suspect most users do a similar thing too.
The thing is, I notice more crashes on iPad 4, too, and it's got the latest and best guts of all the iPads. I do identical browsing on all iPads I've owned, so I've seen the crash patterns over three years, even browsing the same websites and keeping many tabs open. Some of the crashes are related to RAM, and some I deduce are tied to limits among hardware, firmware and apps.