Well, let me spell this out for you, then:
If the camera assumes you can take 12 photos, then it will show that. Then you take one photo. Based on how fast your memory card can store that photo, plus the size of the file, the camera may revise its estimate to that the continuous shooting buffer will be full already after nine shots, since it realizes that it's not getting rid of the already taken shots as fast as is theoretically possible, with the fastest card it can handle.
This happens - it's no magic at all. But if it's the answer to what you observe, I don't know. Just that it could be.