Harriet Pitts' Mother

This is my great-grandmother, Harriet Pitts, and the accompanying photo is of her parents, Robert & Clara Pitts nee Steel.

There is only one record for a Harriet Pitts born in Sheffield +/- 5 yrs of her known birthday, so I sent off for her birth certificate.
When it came, it recorded her mother's maiden name as Sharp!
But there are no marriages for a Pitts and Sharp - just Steel!
So, to try and resolve this, I sent off for the birth certificate of Harriet's sister, Ada.





Ada Pitts' birth certificate arrived this morning, and it records the mother's name as Clara STEEL!

As Robert and Clara married in 1875, two years before the birth of Harriet, I can only think of two possible scenarios:

1/ either Robert had Harriet via an extra-marital affair, which would be a great coincidence if the mistress' first name was the same as the wife's

or 

2/ there is a misprint or transcription error on the birth certificate.