I think..... it is because the Palace ( protocol Office) said she is
1952 - round two over HRH and the Duke of Windsor, Jowett, then Lord Chancellor, said - "look basically boys and girls, it is in the power of Monarch to say what, when it comes to titles of the Rt Royal Rolicking Rowdies ( family that is)"
which was rather different to Round 1 - Jowett again KC this time 1936, where he said an abdicated King went back to what he was - HRH Prince, so the wife was HRH Duchess
and this rule ( the old one) is what they relied on, to give Lilibet and Archie, Princes titles - grandchilren of the Monarch (CIIIR that is!) were princes a/c Letters Patent 1917
it is a funny old worlde, innit? on AB