Because having three letters doesn't sound as good as two
Technically they should be YVF if they were to accurately fallow the tri-service designation system. Then again if they were to actually fallow that designation system correctly, then it should be YFV and all valks would actually be FV-X's since the V indicates vehicle type (other than fixed wing) which should come last. Proper letter order goes status prefix (Y, X, N etc.); basic mission (A, F, B etc.); and finally vehicle type (H, V, Q).
also there's the tendency to skip series letters (the letter after the number) like going from A to d then to J then to S