This is probably Sony being cautious. Same case with MS.
They are probably not using this amount of RAM for the OS but gonna see how well they can optimize and what new features need to be implemented over the first year or two.
They will probably slowly allocate more and more to games.
The thing that they absolutely can't do though is give too much memory to games and then have no options later in the gen to add features which is exactly what happened with the PS3.
I was really expecting only 2gb to be used for the OS for both MS and Sony...seems like both have more ambitious plans for the OS. Either way i don't see this as a big deal...probably will have a very limited impact on games.
Still makes all the Sony fans who were using the whole 7GB > 5Gb argument look kinda silly now Even though that was never confirmed.