Is it Guarantee to Get Your Role in Dynamic Queue?

For the past 15-20 games of dynamic queue, 100% I get something else besides my main role: mid. I am not complaining nor being angry about it (since support is also my main) but I find it unfair to see that someone else gets mid but I don't. What's even more strange is that the people who got mid in my games have their first letter of their names ahead of me, meaning that they get mid if they're a, or b, or even l, while my first letter of my name is "u." So what I mean is, I can get mid if someone else name starts with v, w, x, y, or z. Is that the case, or is it just random? But anyway, are some of you getting something not what you wanted, although your first position is "mid?"