Not absolutely uniformly, but there is consistency there by and large.
For 19 and 92, you'd have to ask George VI and those who advised him why the same criteria weren't applied as for 617 and 120.
Actually, for 19, you wouldn't, since they fall victim to the simple fact that by the time these decisions were made, they had the 25 years accumulated service to get their standard, so couldn't be awarded it early...
Anyway, this is about 1(F), and it's difficult to dispute the choice of that numberplate.