5DsR Bufer wierdness

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The number of shots you can take, that's shown in the viewfinder, is an estimation/approximation of what the camera expects to happen. It's not magic truth.

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?