I think looking at the samples, on the A900 update, aka raw..it is very obvious what is going on.
I have raised this in the past, and will continue to do so. The reason ACR produces what appears to be, a larger grain pattern, or what I would call splodges..rather than a tight grain effect, is the apparent base level NR going on in the software. I see this effect in my MRW files too, though as they are less pixel dense, it's not as bad, but still present.
I find it easier to remove noise from a tighter grain pattern myself (if indeed you want to remove it), and I would suggest adobe consider allowing users to switch off the noise reduction effect. This extends further than pure pixel peeping, whilst it's no big deal for smaller prints, for larger ones it is.