My 5Ds does exactly the same thing. I'm using a Sandisk 120MB/s card though.Buffer 12, as soon as you take a photo jumps to 9 and pretty much instantly jumps back to 12. However if you use the entire buffer and end up with 3 left for example it will show all inbetween numbers before getting back to 12.
To be fair I came from a 5D2 so the speed and buffer where a huge step up anyways and I never paid any attention to this