Perhaps she married a man with the same surname?
That's a possibility. The husband/father or whoever he is, is not in the census...just Helen and her 7-yr-old daughter Helen.
I was given a photo of her from a niece in 1987. Her image had been cut out from whoever else was in the original photo, so the husband may have left her and she kept her maiden name as well as giving the adopted daughter her maiden name??
This is the last record I can find on her...she was in her 40th year in 1921.