Tolerances when centering a ceiling fan
Take that box back and get a box that attaches to the joist

It sounds like you have the wrong box for the job -- a hanger-bar type fan box like yours is designed for putting a fan between joists, not adjacent to them.  What you want instead is a fan-rated box that mounts to the side of the joist -- ask your electrical supplier for a Raco 294 or equivalent (depicted below).