In some years, a lot of children were simply recorded as Infant of X. X usually being the father's name, though occasionally it was the mother's. It appears Ancestry indexed all the names as the mother's name. (I am assuming all the Johns, Jims, and Williams are fathers.)
For example, in 1906, out of slightly over 23,500 records, approximately 17,700 were recorded in this fashion. (Roughly 75%)
In its description of the database, Ancestry does admit:
In some cases, where the names of the parents are provided, the parents' names were recorded incorrectly. Occassionally (sic) the mother's name was recorded in the father's place and the father's name was recorded in the mother's place. Therefore, you may need to switch which fields you put the parents' names in, in order to produce results.I might argue with the 'occasionally' and the 'some cases.' Taken overall from 1903-1997, it's probably a small portion of the database. But if you are researching the earlier years, it appears to be the majority of records.