Well, before they left the RF they were the modern, unstuffy couple who were going to revolutionise the old institution... and their child had untraditional names. They weren't going to use his title.
Then they left, and it became apparent that their main selling point was not their modern, unstuffy selves, but their connection to the RF.
Strangely enough, the child born at this point has names very much connected to the RF, to the extent they used QE2's family nickname, not the formal 'Elizabeth'.
And a year or two down the line they want to use the children's titles.
No doubt a coincidence, and they just happened to like the name. And change their mind about titles.