I've gone ahead and moved the thread to the multiplayer section to make it easier for our multiplayer team to take a look at and listen to your feedback.
But just as a short explanation of how it works -- The matchmaking system tries to find a suitable match for everyone queuing. So if you queue as a premade, it will try to match you against another premade, and following that it will look for smaller and smaller parties until eventually filling the match with solo players. Unfortunately, this does mean that sometimes you will be matched against a full premade depending on how many players are waiting in the queue and how long they have been waiting there.