I've ran into this same thing a couple times which has given me extreme grief in tracking down my 3rd great grandmothers lifeline after I was 1st able to figure out her last married name working from 1900 back through others in her family. Earliest records 1850 Guildford,VT show her as MaryAnne Easterly widow from NY with son William born in VT living with the mother in-law. Next Helen M Easterly a housekeeper with son William living with an elderly Benjamin Mann in 1855 Troy,Ny. From there 1860 Helen Mann, she has a few variations of Helen M, Hellen, Ellen ,Marian & H Marion. After many years of not wanting to just fill in blanks I deleted the line took a notebook and started over backing assumptions with links to others and common names as with her sister inlaw who died Jan and her daughter with Benjamin Mann found in multiple places before I came to the same conclusion. The same holds for her daughter were she's found as Jane or Jennie right up to her mother Helens obit in 1900 Troy/ Lansingburgh, NY were they refer to her as Helen along with her mother.