Mmm- I agree.
However, she was with the Johnsons who were possible relatives and they had been married since 1853 and apparently no children.
I wonder,even though she was a servant, whether that had any bearing on it.
There may be other Sarahs in the extended family.
Your Sarah is the only one in 1851 census but there is a Sarah Jane in births in addition to a Sarah and there is a death for a Sarah.
On the other hand, there is a Myra link.
There is a marriage on Family Search 1876, Hulme,Manchester Sarah Griffin (father Henry) and William Taylor.