Could be Jane Robson was married before and that was a child of hers from a previous marriage , or she was not married at all and William was an illigitimate child of Jane's. You would need to get the marriage of Abraham to Jane to figure that one out.
Robson was a very common name in the area, not quite as bad a Smith or Thompson. I have quite a few Robson's in my family unrelated to each other.
Janis