Why is what you are registered with in some database in some government database more of a real name than what your friends call you? If I know you under one na

Why is what you are registered with in some database in some government database more of a real name than what your friends call you? If I know you under one name and your kids calls you something else, which name is the real one? People have different roles and behaviors in different situations including being called different names. This may be inconvenient for states and other large organisations.