Why can 3-man queues guarantee 2 people primary, but a Duo can't guarantee 1?

Let's call the roles A B C D E Call the players X Y Z

In a duo queue of X and Y, players are required to choose a total of 3 roles. i.e. you cannot do: X: A B Y: B A So the best they can do to try to get role A for X is: X: A C Y: A B However it's still possible they get assigned B and C rather than A and B, or A and C.

Now take a trio queue of X, Y, and Z. The players are required to choose 4 roles. X: A B Y: B A Z: C D

Now X is guaranteed A, and Y is guaranteed B. Only Z may get his secondary role.

Why does it work like this. Doesn't it seem imbalanced?

With dynamic queues, a three-man already has a significant communication advantage with no matchmaking penalty (according to Riot when they released the queue), and they can also get the advantage of having 2 players guaranteed their primary roles (and the remainder all still have the same chance at primary/secondary as the rest of us.