What contributes gate charge in a MOSFET besides gate-source capacitance and gate-drain capacitance?
A MOSFET is modeled as three capacitors as shown below.

As such the total capacitance is rather more complicated than just the sum of the Cgs and Cgd.







simulate this circuit &ndash; Schematic created using CircuitLab

In fact in some MOSFETS there are other capacitances due to the construction. 

Ultimately the capacitance also depends on the state and voltages involved.