@Kronous It's not all that suspicious, actually. A mom can easily tell you who she thinks your dad is and it's not impossible to not know your mom's real name when you're young, especially when you're like 5. If the mom isn't overly familiar with people, or you aren't around her dealing with friends/family or her regular shopping trips then it's easy to only recognize her as "Mom" and nothing else.
You usually learn your parents names when you hear other people using them. Until then your brain may not make the conscious connection that they have other names. You might recognize that since other people have names, your parents should too that might trigger your question to ask them but it all depends on the intelligence and exposure of the child and how much they actually pay attention to the information around them.