[El Gato]

Or mom got married/re-married after her son was born. Or took her maiden name back after a divorce

Or the mother continued to use her maiden name after she was married. That's not all that uncommon these days.

[Tired of Taxes]

Or the mother continued to use her maiden name after she was married.

That's what I did. Socially, I always gave my husband's last name, but legally I kept my maiden name, thinking it would be easier than changing all other documents to my new name. What a mistake! Whether we were buying a house or taking our kids to the hospital, everyone was confused.

The questions I got:

"Are you REALLY married or just saying that?"

"And who are you? You're his... MOTHER? Really?"

It never helped that my kids look nothing like me... lol.

All that said, it is possible that the mother is a single mother who gave her child the father's last name. That's quite common.