Census duplication

[Frodo]

I have been researching Mary Ann Billingham (abt1841 on BCC ) and have found that on the 1871 Census along with her husband Frank Bullock she is living in Walsall with four children including Frank age 5. Looking at her father Benjamin Billingham (1818-1896) in the same Census, living with him are his wife Charlotte, four children and his grandson Frank age 5. There is a black mark against Frank's name. Does this mean that Frank was with his grandparents at the time and listed with Frederick and Mary Ann to give a complete picture of their family?

[gardener]

Unless the households were done by the same enumerator, I doubt if the mark has any meaning like that. Aren't most of the marks added by people who were counting and compiling statistics?
It is not uncommon for people to be added in even if they were not home on the actual night.

[Frodo]

Thanks, that is what I suspected was the case