From my understanding the additional one gigabyte of RAM is "hardwired" to the camera, so the OS itself can't really access it, but the camera (or specific camera apps?) can. This would mean even though the 1020 is listed as a 2 GB Windows Phone device, it behaves as the usual 1 GB phones like 920 or 925. Yet the camera takes advantage or even need the additional gig of RAM to work better or even just work.
The 1520 or 930/Icon on the opposite have 2 GB of "traditional" RAM which can be accessed and used completely by the system, because the inferior camera module doesn't seem to need dedicated RAM.
Before you request any sources of that statement: I don't have any at hand, this was stated a couple of times in the past here by several WPC members, I also remember a topic where a 1020 only showed 1 GB using a hardware information utility. People explained this is normal, because the OS can only see one gigabyte, because the other one is exclusive to the camera.
I also remember I read a similar side note on a review of the 1020, don't know though if it was here on WPC or over at All About Windows Phone.