They can only do that because they only have roughly 150 seats installed, with 'only' two aux fuel cells fitted in the aft cargo hold. Also the extra doors on the -ER are deactivated and plugged to reduce maintenance requirements.
Concerning MZFM - thats a matter of certification, the 900ERs I am familiar with have a MZFM of 67721kg while the -800s have 62731kg.