I read Roberts' section on the Girls of Great Britain and Ireland tiara and in footnote 3 on page 136 he quotes from description in Garrard's ledger which states that the jewel could be used to form a "small Crown Comb &c &c".
I have partially convinced myself that the jewel Queen Mary is depicted wearing in the portrait is the "small Crown Comb" If so, then I think that smaller elements from the back of the tiara have been placed to the side of the central element.
The use of some of the smaller elements from the back of the tiara could also explain some of anomalies I see in the coronet form worn by Queen Mary to the Devonshire House Ball.