Inconsistent Quinnins

On my Martin Quinnin's birth certificate his birthdate is recorded as 15 September 1881, and he was registered on the 28 October.

An extract from Martin's birth certificate.

Martin was baptised on 13 October only fifteen days before he was registered, yet his birthdate is written as 2 September. So which is it? I think it's quite interesting to see a thirteen day gap, not simply one or two which I think could be forgiven.

Martin's Catholic baptism entry.
Under Queenan.

Everything else is correct, the names of his parents and the family's abode of Melton Terrace.