Should my fan have been installed off center?
Do you have it in the contract that the fan in centered?  or a drawing?   You as the homeowner and the payer of the bill have the right to have the job done to your specification.  Get them to come back and put the fan on center.  Would you be alright if the fan was off center in that panel in order to center it in the space?  That way they wouldn't have to pull and re hang the whole ceiling.
Good Luck!