Ranked Promo series? Clarification please.

Just out of curiosity, but every time I hit my promotional series, I seem to have unfair teams. Using lolnexus to check the rankings of both sides, normally they are pretty balanced in normal ranked games. However, in my promotions there always seems to be a huge gap with like lets say plat 3 to plat 2. I would have a fair balance of plat players on my team from divisions 5 to 1 or rarely a diamond. In series however, the enemy team seems to just be all diamonds. If it is possible, I would like a clear explanation of how the match making works specifically for promotions. I really do not want to hear about mmr because I have seen this countless times... roughly like 50 times because thats how many times I have done some promos. I feel like its just unfair, but I would really like some clarification.