The Watson surname appears for Agnes on a census. That often happens for stepchildren. What you see for 1841 to 1901 censuses are the enumerator's form, copied from the individual house holders forms.
It depends how fully the head of household had completed the form. If he just put his own full name, then followed by the forenames of wife and children / stepchildren, and did not differentiate between children and stepchildren, just listed them all as children, the enumerator would by default give them all the same surname as the head of household.