hi Zahid,
Uplink frequency is from MS to BTS and Downlink frequency is from BTS to MS. BTS has o/p power of 20 watt and MS has o/p power of 2Watt.BTS has already higher power than MS, so in order to maintain syncronization between two Uplink frequency is lower than downlink.
I am not sure this is the right answer of yourquestion so, if u came to know about some other reason for this, then plz tell me also.
Regards
Sachin