I believe the dates and jewellers that the Royal Collection are referring to, actually concern the necklaces that the Queen wears in the miniatures and portraits. Not the circlet, which is undoubtedly George IV's Regal Circlet.
Rundells 1839 refers to the Turkish Diamonds, which were a present from the Sultan of Turkey and made into a necklace and earrings by Rundells in 1839, she subsequently wore these to her wedding.
Garrards 1858 refers to the necklace and earrings we now know as Queen Victoria's Collet Necklace, which were made in 1858 by Garrards using diamonds from the Queen's collection, to replace Queen Charlotte's diamond necklace that was lost in the Hanoverain claim.
These dates and jewellers do not refer to the circlet. It is just poor grammar and sentence structure on the part of the Royal Collection that leads to that confusion.