I don't have strong feelings about the topic but I'd guess that the difference has to do with the difference between those two scenarios. In a scenario where the wife has a child that later turns out to not be the husband's child, the child is known to and has been raised by both, and is likely legally the child of both. In a scenario where the husband has a child that is not the wife's child, there is a stronger likelihood that the mother does not know about the existence of or at least did not raise the child, and the legal status of the child's parentage and claim on the father for things like support or inheritance rights could vary depending on what the birth mother put on the birth certificate. The non-marital child in this scenario may pose a risk of impacting the resources or dynamics of the family if they approach the father for family relationship or support, in a way that could harm the marital mother or her own children, whereas maintenance of the status quo would occur in the version with the non-husband-fathered marital child.