Champion Select, Riot Please Respond

Why do people get secondary roles when their role picked doesn't interfere with others on their team?

I was just in a champ select that i qued Top Primary and Mid Secondary. I ended up getting Mid.

Our top laner asked me if I would trade lanes with him..........I told him yes and that i actually wanted top but got secondary role.

He told me he qued up for Mid Priimary.

So if he had primary mid and i had top primary why does the game give us both secondary roles?