Hi,
"Pin reading". What measurement device? What value us shown?
* "average" is just the DC offset, it may be the same value with or without RF
* "RMS" value can be with and without DC.
I'm not familiar with this application...but the IC is called "phase detector". It may operate similar to a logic "XOR" circuit.
Then the RF input signal may be constant, but with varying phase shift, the output signal will vary.
Best is you read the IC datasheet ... or go to the manufacturer's internet pages, and look for relating application notes to get more detailed informations about it's operation.
Klaus