The same thing happens on hotslogs, your first 1-2 games of the season swing your MMR as much as +100 to +300 points. After ~5-6 games you only move 50-60 MMR points, and after 20+ games you only move around 10-30 points after a win or loss.
Since I assume Barrett and the new owner (Zam Network) have modeled their algorithm to reflect average MMR swings in the HotS rank system, I would say their calculations are very accurate especially for players who upload at least half their matches (or even all of them).
It is sad that the first few matches of a season matter so much, because this is a 5 player team game where you could just get very unlucky and have an AFK or feeder in your first or second game. That should not count for nearly a full league tier of rank points.