I know I did the test correctly, before you ask *lol* I did it both ways - measuring from coronet band to the middle of the knee and converting inches to hands, and measuring length of fetlock to elbow and doubling it, with the results the same either way. But I have always been under the impression most foals mature somewhere in the range BETWEEN the height of the dam and height of the sire. Freyja, the dam of both (Finn and Fiona are full siblings) is only 15.2-15.3 ish HH. The sire of both is a Shire stallion standing at 18 HH even. I would expect, then, that both Finn and Fiona would mature in the 16-17 HH range max. Maybe they will, and just be exceptionally leggy? It just leaves me scratching my head as to how they could both be a fair bit taller than their already tall sire. It just seems like a case where this has to be terribly inaccurate.
Finn, 15.1, 15 months (note I'm 5'9"):
Fiona, 13.2, 3 months: