The answers to the whole section about marriage and children on the 1911 census are rife with errors, some accidental some deliberate.
The most common error appears to be listing the answers next to the husband rather than the wife.
Other errors are answering it for widows, lying about years married or being married, including illegitimate children, including children from previous marriages, including miscarriages.
My 3 x g grandmother was living with her daughter and they were both widows but they still answered the question and both also appear to have included miscarriages in the total number of children born alive. My grandad and his first wife also appear to have done the latter.
My 2 x g grandmother was with her third husband and they had no children but with them were children from both her previous marriages as well as his previous marriage. They were all included in the number of children from present marriage, though the ones not with them were not included. And also listed with the husband's surname, took my a while to sort them out.