Legal Question in Family Law in California

Probably because of another doctrine, such as parentage by estoppel. You don't provide any facts for us to explain it to you.

There are many possible reasons, but the most likely one is either 1) you were married to the mother and still living together when the child was conceived - in which case the law makes you the father unless the biological father comes forward and asks a court to determine he is the father and gets a judgment that he is, OR 2) you have never gone to court, filed a paternity determination case, obtained a court ordered and authorized DNA test and submitted it into evidence at a paternity determination hearing, to obtain an actually legally binding judgment that you are not the father. If it is not one of those two reasons, then you would have to provide a LOT more detail about the situation for us to even guess at why you are considered the father for child support purposes.