I have wondered about that as well, there definitely seems to be a lack of discipline in the registers of many countries nowadays, especially with 4 letters or a mixture of letters and numbers, or just numbers after the suffix.
A Turkish AF A400 went over London in the last half hour pretty lowish for either Northolt or Brize. It sounded different to the RAF A400's, perhaps the engine choice is different ? Anyhow, it went into Farnborough, which seems a strange choice, but I'm sure that they have good reasons.