Formula

RankPoints = Win% * (CrossPoint + (1 - CrossPoint) * (1-((1-MIN(Limit, GamesPlayed)/Limit)^x)) + NonConcedeBonus

where x = log (a/(1-CrossPoint)) / log(1-(Target/Limit))

Crosspoint = 0.2 - Was 0.4. This is a change made at the start of S11 to accommodate the lower win rates of T3 races
Limit = 42
NonConcedeBonus = (played - concede) * 0.02 - NB concede = 0 for the moment.
a = 0.05
Target = 28

WTF?
For those who looked at that and went "wtf?" here's a summary:

You are basically rated on your win percentage (win% = (wins + draws/2)/games played) BUT...

1. You don't get the full "value" of that win percentage until you hit 42 games (one game per day). The amount you get is scaled down with the games played (almost - there is a slight adjustment from the linked formula) in accordance with the curve you saw such that if you only play 28 games (4 games every 7 days) you get 95% of that value. That is in order to encourage people to keep playing their team and not sit on good but low games played records.
2. There is a bonus for playing more games. It's not much - 0.02 points per game played (bear in mind a 100% record would get you 100 points at 42 games) - but enough to create a difference between two records with identical win percentages and different games played.

So,
at 14 games you get ~71% plus 0.28
at 21 games you get ~86% plus 0.42
at 28 games you get 95% plus 0.56
at 35 games you get ~99% plus 0.70
at 42 games you get 100% plus 0.84
for every game thereafter you get an extra 0.02

Example records:
48-0-12 = 80% win% * 100% modifier + (60 matches) 1.2 = 81.2 points
35-2-7 = 81.81% win% * 100% modifier + (44 matches) 0.88 = 82.69 points
14-2-0 = 93.5% win% * 76.1% modifier + (16 matches) 0.32 = 71.71 points
24-2-4 = 83.33% win% * 96.6% modifier + (30 matches) 0.6 = 81.11 points