Why calculated Fps is more than the actual Fps?

I have created an application in Android using CamerBridgeViewBase to access camera. The application detects face and eyes in the captured frames. For some reasons I added a method to calculate Fps. Now my problem is, whenever a face is detected in the captured frame. The Fps will be almost same as given by enableFpsMeter(). If I focus my camera on some other areas such as plain wall the FpS will be more than the actual FpS. Why this is happening? How to solve this? I calculated FpS in the same way the opencv used in FpsMeter object to claculate FpS.