Sometimes it's the baptism entry which is the fib.
I've just transcribed a 1933 baptism with an unusual surname. I looked for the birth registration to check spelling.
I found the birth with the mother's maiden name rather than the unusual one. In 1939 the father seems to have been 100 miles away and married to someone else.
The child seems to have married in the late 1950s with an extra middle name, matching the father's surname, so may have known about their parentage.