It’s simple : the TS’s which are using the BCCH frequency will be sent at maximum power.
For one MS receiving DL TCH with DL PC : once in a while it will receive a TS with max power and other times, it will receive a TS with “controlled” power. The average DL rxlev will be “increased” by this mechanism.
There is, therefore, a parameter that allows the MS to NOT take into account the TS “with BCCH freq”in the averaging. (in alcatel, it’s called PWTC, if I remember correctly)