I think it's a combination of misunderstandings, white lies and full-on lies.
One greatx2 grandfather keeps giving different birthplaces in the censuses - all close, but not one single location - so I've never found a baptism for him to pin down his parents.
I assume that he lived with his parents in various places and never knew exactly where they were when he was born.
A great grandmother consistently gives age matching birthdate of c 1850/51 - but I've never found her registration or baptism, although there is another very tempting person of the same name born in 1846 who might be her. Did she just not know how old she was? Did she match her age to her 'husband'(s), neither of whom she actually married, so far as I can find?
Who knows.....