Not writ in stone, by any means, but its not unusual for a birth to be registered, when the parents for whatever reason are not married, with the father's surname as a middle name.
IF this is the case I would say that the Robson was the mother's name and to give the children their father's name it was registered with the father's surname as a middle name and then used as the surname form there on.
I have an instance on my tree where this happened, the lady left her (nasty piece of work) husband and lived with another man , declaring themselves on census returns etc as 'husband and wife". Many years later when nasty hubby died, they breathed a sigh of relief and nipped to a registry office and made the union legal.
it could have been the same for either party, not necessarily the woman, but the man could have entered into a marriage that was so unhappy he left.
Remember that back in those days divorce was not an option for the everyday folk, you had to be quite rich (and extremely thick skinned) to go through that process.
It may not be the case for your scenario, but its something to stick into the 'possibilities' box.
Boo
