Yes, thanks, Janette.
Well done finding the bapt, which is in the IGI. Not found (by me) in the FHS births, though.
It looks like a transcription error by the IGI. You can double-check these on the parish register filmed copies which can be viewed for a small fee through any LDS Family History Centre, and you might find William as well. Look in the phone book under 'Church of Jesus Christ of Latter-Day Saints" for the closest one - there are lots in NZ - and open to all.
The Marown film number is 0106191 [1849 - 1883] "Item 2 Parish register transcripts of Kirk-Marown. Church of England. Parish Church of Kirk-Marown (Isle of Man)". It has "Christenings, marriages, burials 1849-1883" so you might find other family events there as well, with additional information such as occupations and addresses.

Sue