Let me spell this out for you:
1) Turn the camera on. Set it as desired. Observe buffer to be 12 shots.
2) Take one shot. Observe buffer to be 9 shots.
How does a 12-shot buffer dip to 9 on just one shot? Or, phrased differently, how screwed up does the estimation logic have to be for one shot to magically eat up 25% of the buffer?