Then they shouldn't affect the amount of actually obtainable missions.
I very much know what there is to come when I get elite. I don't need to have 12/14 missions unavailable as a reminder from RNG.
How do you know what comes when you get to Elite, without being there or seeing these missions?
Where does it say that you get less obtainable missions?
Where has it been said there is a maximum number of missions that can be rolled?
Where does it say that the set of missions for attainable and unobtainable are separate or combined?
Furthermore, why should you only be able to roll obtainable missions? That doesn't really make sense for a 'believable' world unless you have all the skills and equipment those missions require.
Honestly, I don't understand why you would even play with the RNG in the first place, I go to a station, see what missions are there and if there are no applicable missions I move on or come back later. If you're in the game of constantly spamming the BB and logging in and out just to get some high paying missions why don't you just ask FDEV to give you a button that gives you credits. There is more to the game than accumulation of wealth and you would probably enjoy the game more if you focused less on gaming the game to max out your credits and more on just playing the game as it presents itself to you.
You're getting frustrated because it's not giving you the max payout mission type for your career on every occasion but I guarantee it would frustrate you more if each and every possible mission was ALWAYS at the station. You would be asking for "variety" then, I have no doubts.
Digressing slightly:
I was in Wolf 406 last night and Transport Co gave me a 200 Cr fine for clipping the letterbox on exit after dodging some pilot who was racing in on the wrong side. Will not be doing business with them again lol