We investigated the reason of the gap between the measured and calculated resonant frequency.
However, we cound not get specific solutions explaining this problem.
Thus we changed our storategy.
We "sweeped" some capacitances. 
After some try and error, eventually we could find a good capacitance and tune these RF QPDs' resonsnt frequency to 13.78 MHz.
These RF QPDs were took to IMC REFL table.