jnr,
no, the rxlev should be *in average* the same between both antennas.
At one precise moment, you might see ultra-quick variations of 10dB, but in average, the difference should be 0dB. Otherwise it means that there is a HW problem somewhere. I would suspect the feeders first, then the BTS Combiners, then the TRX, then the antenna HW.
Antenna installation, if badly done, could also lead to a difference. I’m thinking about two horizontally separated “V-pol” antennas. On antenna facing one direction, and the other antenna 6m away, shooting on an obstacle (satellite dish…), or not facing the same azimuth, etc)
In case of Cross-polar antennas, antenna installation cannot play a role in this particular problem.
Cheers
pix