Check out the possibility of more than one marriage. I don't know where you are looking, but in the poorer communities, say Ag Labs and Miners, death was common and the surviving partner could marry again very quickly as combining households was often the only way to pay the rent and look after the children.
That often led to multiple surnames for women, who might marry listing their previous married name but record their MS as well, or sometimes instead. My own gr.gr.grandmother married in 1846 so both names were clear, showed MS on 2 children's births, was widowed in 1850 and then gave her MS on the 1851 census, though her children had their father's surname. When she married again she used her married name, and all 3 names ended up on her death certificate.
You're right - it is confusing!
