There was no legal reason why she couldn't have taken her stepfather's surname at or before marriage.
No formal name change required.
The census only tells you what information was given to the enumerator. In both these cases she would not have been the informant. At marriage she was the one listing her own name. Perhaps she decided she'd rather have the same name as her mother, stepsiblings if any, etc. Would be interesting to see how she is listed in the 1921 census.
