Why would someone use a wrong birthdate despite knowing his correct one?

[coombs]

Wonder if this also applies to someone born in 1964, much more recent? A family friend this is, he said he was born on the 8th Nov 1964, he was the son of Jamaican immigrants, born in Northern England. But he died of a rare lung illness in his 40s and the funeral service card said born 1st September 1964. His birth appears in the last quarter of 1964 in the GRO indexes though. I may never be 100% sure which was the correct birthdate.

[Biggles50]

Its not unusual, my second Wife said she was four years younger than she actually was.

She came clean after we had known each other a couple of months.