Long exposure NR (LENR) stores 2 frames for each image, the 2nd frame is a "dark frame", similar to shooting a frame with the lens cap on.
It would be interesting to see if you really only get 6 or 7 frames with normal shutter speeds because LENR supposedly only occurs if the shutter speed is slower than 1 second (or so-forget the exact number). Just an idle thought in the interest of science . In other words, it is possible you would actually get double the "rxx" value for normal shutter speeds. Unfortunately I don't have time to do science experiments today .
My D700 indicates a few less frames than Thom Hogan's eBook numbers, which were based on his usual detailed testing. It has never been clear to me why that number varies by a few frames. All the cameras of the same model should have the same identical buffer capacity.
Just to say I cannot explain your r15 vs my r17. I just knew R06 was "too low" for normal settings. (I do not activate LENR by default because I do not believe it is generally necessary with my cameras but that is another debate!).
I think DPR's r12 is low, based on my recollection of discussions here, but I never studied their test findiongs.