questions about the Dynamic Queue

So tell me how my First Preference is Jungle (and thats my main role) and i put my Secondary as Mid (second best role), now.... i can tell you... that 95% of the time i get my secondary role, now dont get me wrong its not bad for me, but through having it 10 times in a row is really irritating. Alot of times those that would supposedly get my Primary role are really bad players, sometimes gameplay wise and toxic wise or both. So can anybody explain to me how the Dynamic Queue works in finding which people want to play what and how it selects people for that place. Do i make any sense in my question or now?