Was it the same surname though as I am assuming you checked mother’s maiden name on the FindMyPast or GRO index before ordering?
Here's the weird thing. The father's name is given, one William Jolly, no occupation yet in the index the mother's maiden name was given as -.
Which made me think I was on the right track for the illegitimate birth.
On the BC her name is Grace Pritchard, a domestic servant, which is unusual in itself to see an occupation. Today I fiddled around on the indexes and the birth is also registered as William John Jolly, the same volume and page number. It doesn't really matter as it's not who I had hoped it would be but I've not come across a double registration before.