Without looking at the instructions, I guess the definition of "head" may have just been the most senior member of the family present on the night. I came across an entry in a different years's census where the head was a babe-in-arms accompanied only by his nurse and a few other servants, the parents presumably being away.

I also came across a soldier sleeping it off in the cell of our local nick (singing not mentioned), relationship to head: "prisoner", so he would have been registered if there. Also counted were the sergeant and his family plus a dozen or more constables, presumably actually out on the beat -- considerably more than we have today.