For allocation on 1800 band, calculations are done by BSC to see however or not the MS is receiving sufficient level on 1800 band for allocation (others calculations are done too). This is done by appliyng an offset to the rxlev received on 900 band like this:
Rxlev900 band = (approximately) Rxlev1800 + Offset.
This offset is theoriticaly 10 dB but it could be in reality greater or smaller than this value. Optimizations guys have to tune this parameters to better fit the environnement and traffic balancing purposes.