This question was asked hundreds of times by old burggers like me. The answer always given was that it was contained in the Annual Pay Warrant issued by Act of Parliament and anyway "when you joined the Club you knew the rules", followed by the "you are on Officer not an OR" argument.
Not sure that would stand up to scrutiny in this day and age but in those days you saluted smartly, turned crisply and went before you were accused of being bolshy! (Trouble maker that one Colonel... Yeees, I think you may be right there Charles, make a note! ..... Yes Colonel).
On the plus side however I have heard from serving colleagues that our pension rights were better than they are now.