You do not indicate which country 'Cloud 9' is in, but En-route (Eurocontrol) charges are based on the sqr root of MTOW/50. Some airports charge landing fees based on MLW, some on MTOW. It could even be insurance. Your company will doubtless have found a new MTOW band that saves a bit of moolah somewhere/somewhow on your route structure. As long as the new MTOW is practical and below the max certified by AI and correctly documented (a/c docs and load system) there is no problem. It is a pretty common practice