Because GSM is used on other frequency bands as well.
'900' is a nominal figure. neither the carrier or the LO are on exactly 900MHz.
_Where_ the _receive_ LO is, depends on the particular set design. it may be 'above' the desired carrier frequency, or 'below' it. it will be offset from that desired frequency by "whatever" the IF (or 1st IF, if multiple-state superhet) frequency is. an offset of 10.7 MHz is common,
Of course, the _transmit_ LO will generally be on the carrier frequency, (in _rare_ cases, the xmit LO may run on a sub-multiple, and then be 'frequency doubled', or 'frequency tripled', before transmission. This complicates the design, and for that reason is "unusual", at best.)
Essentially, the number was picked because it is a "convenient" round-number _name_, that is "approximately" accurate.