Can someone explain match making?

I do complain about match making but have never figured out exactly how it works. I understand the WG does not want long que times and would like a sense of competitive game play. I also play NA server so it might be different in other regions. During the main hours that I get to play it list 12-14k players on. With that many players available it seems simple to pretty much have tier for tier games without extremely long waits. Especially tier 6 and up. Since this does not happen what data is actually used for MM? I have tested several scenarios but was not able to compile enough stats to prove or disprove any one theory so far. I have played on a friends account that is fairly new and very low scored rating wise. He also maxes out at tier 8 tanks. I did notice the first few games all being same tier but after 2 days of averaging 7000-8000 wtr days the MM was no longer tier for tier. So does an individual skill level or the skill level in a certain tank have a determining factor? Or is it truly just completely random?