I assume you are getting info from IGI - if the register has 'illeg' then they give the child the mother's name (tho in manx usage the child would be known by father's name) - entries were of the form X son/dau of A B and C D - it is possible that the first child was not indicated as illeg thus IGI placed under father's name - maybe they married after birth of children - you need look at the actual parish reg entry