I'd love to spend longer trying to get my mind round this, but have a very busy day ahead!
But my first thought is that IF this is NOT 'your' marriage, then there should be another William and Mary Ann Farley on the 1871 census, somewhere (unless they died between marriage in Dec 1870 and census in 1871!)
MArriage cert, I assume showed no actual ages, so they could be of any age, and from anwhere.
IF the couple on the marriage cert are not the same couple as the ones you have traced, I would guess they are a cousin or something of William's marrying a cousin or something of Mary's. But someone should be able to find them.
Another thought is to try and trace Mary on previous censuses before her marriage ('61, '51, '41) if not already done.
Perhaps her mum Mary separated from Trayton (good name!!) when she was young and she was actually brought up by her grandparents, maybe using Brook as a surname and thinknig of James as her dad. She marrys, giving these details. But then (!) the truth comes to light, she is told she is not actually a Brooks but a Gaston, that James is not her dad but her grandfather, and from then on she puts Gaston as maiden name on her children's birth certs.
just a couple of quick thoughts - now I have to drive to Nottingham with a son and a car full of all his belongings to take him back to Uni!!!!