Different mother on 1881 census?

My (possible) g-g-grandmother Sarah was in service in 1871, married in 1873 and had 2 children in 1874 and 1879. In 1881 census the children are listed as living with their mother "Elizabeth". Can't find Sarah or husband William in 1881. Sarah reappears in 1891 census back with her children plus one more, still no William. Other than prison or workhouse are there any other things I should be considering for why a different mother is listed?