I found the examples you refer to in the 1820 Accomack Co., VA census by querying for Selby, which brought up page 29. This is quite unique!

A likely interpretation could be that the surname is of the person who freed them. The only way to know is to go to the deed and estate papers for Accomack Co. to see if this is the case.

Another interesting element is to investigate what surnames these people are using later on. Have you looked for these people in the 1830 census? Also, the personal property tax lists would be an excellant place to look for them to see what surnames they are using through the years.

Thanks for pointing this out,
Jane