IIRC, operators are not allowed to chop and change MTOW to alleviate airport fees.
This may be an old interpretation (UKCAA / JAA days), but way back operators could apply for one change (a reduction) in MTOW which resulted in adding an amendment to the AFM. This became the legal MTOW for landing fees etc. An airport etc could ask to see the AFM, and the landing fees charged accordingly.
There should be JAR / EASA guidance on the issue somewhere.