Because the russkis/Sukhoi OKB are crazeee! in all seriousness though the reason why the naming convention for the baseline Flanker and the Flanker variants is just how they segregaed sales basased on more factors than yout typical 'western' aircraft which incluses the different roles, countries, maker (KnaaPo and Irkut) etc etc.
A - Algeria
B = Bolshaya/Bombardirovschik - bomber
D = Dozapravka
I = Indiski - India
IB = Istrebitel Bombardirovschik
K = Kommercial/Kommercheskij (Commercial export Vietnam,Indonesia, Ethipia)
K = Korabelnyj
K = Kitei - China
M = Modifkatsi - modified
M = Moderniziroval / Modernizirovannyj - modernized
also M - Malaysia
MF = Mnogofunktsionalniy - multirole all weather attack
R = rekornostsirovka - recon
RV = Russkije Vitjazi (Russian Knights)
S = Serial - production
UB = Uchebno Boevoy - 2 seat trainer
So for ex. SU-27SK means Serial production, commerical export. SU 30MKA is Mk for ALgeria etc
but even that is not standardized because in the case of MK2 ir just means MKK designated for Vietnam, Venezuala, Uganda etc!
Like I said you need a PhD to undeestand the SU27 designations!
It's hard because we don;t understand cyrilic. for example K means 3 things as can be seen above.
- K = Kommercial/Kommercheskij (Commercial export Vietnam,Indonesia, Ethipia)
- K = Korabelnyj - Carrier
- K = Kitei - China