Census Taker Assumed Entire Household Had Same Last Name?

If the members of a household were not all the children of the same father, keep in mind that the census taker might have simply assumed everyone in the household had the same last name, whether they did or not. 

Step-children might be listed with the step-father's last name, even though he never adopted them at all and they never used his last name themselves. Grandchildren enumerated with grandparents might be listed with the grandparent's last name, even though they never actually used that name.