Yea the whole thing is BS. I have tested it with multiple accounts and this is what I have found.
Dukues: My main account I have had since season 1 - ranked since season 2. Start ranked in season 2 get bronze. Season 3 I am in bronze and when they change the system I am put into bronze 4. Work my way up and end season in silver 5. Season 4, go 6-4 in promos drop from S5 to B1. End season in Silver 3. Do season 5 promos, go 7-3, end promos in silver 5. (start all promos at beginning of new season).
Level second account to 30. Start season 4 in June or July. troll first 3 games (intentionally trying to get bronze). Have horrible stats but go 1-2. Try rest of my games and end promos 6-4. Placed into silver 3. This account dropped to like s4 or something and I did bad in promos so account is still in bronze currently.
Another account. Do promos in August or Sept. Go 6-4 placed into silver 3 - season 4. end season in silver 2 promos. Do promos start of season go 7-3 placed into Silver 4.
One other account in season 4 do promos after season ended. Go 6-4, get silver 2.
There is no doubt that there is some additional calculations being done for peoples MMR. But the point I am trying to make (although somewhat unclear) is that the longer you wait to do your promos the more likely you are to be ranked higher. I believe Riot's theory is since you did not start playing at the start of the season, they allow you to place higher since you do not have as much time to play or basically to get to your real rank. At the start of the season, everyone is hard reset in the hopes that you will want to play and this forces you to play more games to get back to your actual rank.
Like on this account, I dropped from Silver 3 to silver 5 at start of the season. Went on a winning streak and jumped from Silver 5 to silver 3. Have been silver 2 or 3 since. Everyone is hard reset, but if your true mmr is higher you should be able to get back to it quickly. Still think it's bullshit that they spread the handicap on promos throughout the season as you now have first time players who are terrible being placed into silver and then people who have played all year get stuck carrying their noob asses.
Like on my second account where I trolled last season and went 6-4 and got silver 3. In one of my games I was
jungle. I played aggressive and killed the enemy jungler a few times, and had some nice ganks. The emery jungle was freaking out like "omg dude your so good teach me how to jungle". And I was just like... wtf man I am terrible, but here are a few pointers. After we won, the enemy jungle in game chat was like "omg yes silver 3. In my first season! so happy". I know it was just one game but.... I completely demolished this kid. Although he was not terrible I do not think he was ready for the rank he was given.
Anyways, just what I have observed.