Some vicars/curates had a bad habit of filling in the parish registers at the end of the year and went on memory and a few notes scribbled down!
I have always been very dubious of the IGI since I found my great gran on there with her parents, but no other family member what so ever! There were details of their address in 1881, and at that time there would have been several other people living in the house. And then when I acquired the correct data, they had entered my great grand mother's date of birth wrong. However, it is a good site to use as a guide and to give some clues when you get stuck.