Census record - Father has different surname?

Question:

Census record - Father has different surname?

Found an ancestor on the 1861 UK census, but it shows him as the son of parents who have a different surname. They have another son who is older and has their surname, so my guy is not from a second marriage. I'm guessing adoption, but would have thought he'd have changed surname at time of adoption. Was it common/normal to keep original surname after adoption? Anyone have other ideas or similar experiences that might explain it?

Many thanks.

Best Answer - Chosen by Asker: I have similar experiences with the US censuses.

Guess #1, the enumerator was drinking, or got his data from the neighbors, or mixed up the family at #12 with the family at #14.

Guess #2, the older son is the dad's from a previous marriage, your ancestor is the mom's from a previous marriage, and the enumerator forgot to put "step-son" in the relation column.

Guess #3, his relation was something other than son, and, again, the enumerator put "son" by mistake.