I understand what you are saying about the birth place varying.....
I have seen cases where the person giving the information didn't really know the information so gave the latest they knew. It would make sense if he wasn't available (working, etc.) when the census taker came around. They were married in Llantillo Crosseney, so whoever gave the info may have 'assumed' that was where he was born.
Just looked at the 1871 census and it just says Llvanvihangel, Monmouth..... '81 says Monmouth
Between the two census records, there is 4 yrs difference in his age. It is the same John, b/c of location, family names, etc.
First 2 children born in Llantillo Crosseneny, next in Llanbedr and the last 3 or so born in Llangenney.
So, do you think he could be son of George & Mary... Single mother... or is there a Llanvihangel "close" to this area??
By 1871, their two older boys (Alfred & Henry) are living with Elizabeth's mother on Henbant farm (Husband Henry has passed) and the rest of the family has moved on to Llangenney.