Paxboy - sit down! (sorry - but you asked!)
I don't know for this aircraft, but for the B757 I fly, the centre and wing tanks are based pretty much around the C of G, so as the fuel is used, the C of G change is minimal.
Flyin_phil is most likely right - there may be all sorts of cargo in the holds that you aren't aware of, and may weigh much, much more than the pax load.
It also may make it easier for the CC to look after everyone if all 44 pax are in one place as opposed to all over the aircraft - as long as M & B isn't compromised.
Hope this helps.
DW.