uzair,
yes, usually when rxlev is low, the rxqual will also decrease. but sometimes it doesn’t, in cases where the frequency used is very clean (no interferer at all in a 100km radius)
As soon as the frequency is used in the vicinity, the rxqual will drop with the rxlev.
In other words, the rxqual will drop because of “far away” interference, which are usually not taken into account in a standard frequency planning.