Channel to bulk capacitance in inversion
A capacitance is the result of a field between two plates. 

For the MOSCAP in depletion the first plate is the gate and the second the bulk. As soon as the transistor gets into inversion a channel forms and acts like a third plate. 

Since the channel is between the gate and the bulk the field lines originating from the gate will terminate on the channel. The bulk is effectively shielded and instead of a gate-bulk capacitance a gate-channel capacitance is seen.