Yes we're all wondering and are trying to get some official word on this.
The way that different phones and chipsets report RAM use isn't the same, so getting accurate readings from the phones themselves is extremely difficult.
Early research suggests that the Exynos shares RAM between the CPU/GPU, but getting concrete info on stuff like this isn't easy. If that's the case then the reason for 2gb has to be different. The Adreno 225 isn't as fast as the Mali, but it's also not so slow that it can't get the job done. The extra RAM almost HAS to be for a reason other than the GPU, but we haven't been able to nail it down yet. RAM alone wouldn't make up for any performance deficiencies the Adreno has versus the Mali.
Claiming it's because of Sense or Touchwiz is honestly a guess at this point. Sense has had problems in the past with force closing, and I for one haven't experienced that one single time. Multi-tasking isn't the way I would like it to be, but it personally doesn't affect me that much. Not having Sense force close is a good trade, but feels like a band-aid. We do know that Samsung had ICS source longer than anyone, so HTC could be tweaking it as we speak for "Sense 4.x" or whatever it is.