Hmmm, let's leave the troll under the bridge and continue our discussion...
That Royal registration document is giving us even more info than I at first thought. It's a goldmine of info in fact if you apply a little deductive reasoning...
As I pointed out earlier neither William, Kate or George have surnames though Kate used to have one which is clearly written as MIDDLETON.
What has happened? How can her surname have vanished. How come none of them have surnames?
Well we know their family name is Windsor, so the obvious conclusion is that surname does not equal family name. None of them could supply a surname because they simply don't have one.
So what is a surname? Well according to that document it is the last name of the legal fiction. Hmm that's news to me but I assume it is correct as it is on an official royal document. More on this later...
So how come Kate lost hers? As a commoner she had a legal person (strawman), was bonded and was surety for the national debt via joinder with her strawman just like the rest of us. When she married William her social status was raised a few notches to that of princess.
How can a princess be bonded or be surety for a debt? The horror! And so her strawman has simply been scrapped. No more bond, no more surety, goodbye joinder.
The Royals have no legal person or strawman to begin with and those who marry into the family have their strawman killed off. Probably applies to all nobles in the country too - dukes, earls etc.
Now let's check that word "surname" in law dictionaries:
Black's 1910 2nd Ed
SURNAME. The family name; the name over and above the Christian name. The part of a name which is not given in baptism ; the last name ; the name common to all
members of a family.
The name over and above the Christian name? What the hell does that mean? From my experience older dictionaries are better. I wasn't disappointed...
Bouvier's 1856
SURNAME. A name which is added to the christian name, and which, in modern times, have become family names.
2. They are called surnames, because originally they were written over the name in judicial writings and contracts. They were and are still used for the purpose of distinguishing persons of the same name. They were taken from something attached to the persons assuming them, as John Carpenter, Joseph Black, Samuel Little, &c.
So a surname is not a family name. The Royals clearly know that and so don't use one. The omission is not a mistake, it is deliberate and correct as we would expect of the Royal Family. It's the rest of us making the mistake.
Now let me draw your attention to "They [the surnames] were taken from something attached to the persons assuming them..."
Hmm now what do we, as persons, have attached to us? Well in the past it was literally anything. Our occupation (carpenter, cooper etc), our location etc.
But nowadays it's, wait for it, OUR STRAWMAN, OUR LEGAL FICTION.
And this is why Kate's maiden surname is MIDDLETON and not Middleton. MIDDLETON is a surname, an attachment, the last name of the strawman. Middleton is simply the family name.
What do you reckon guys? Do you follow the reasoning?