You are right, the formula has to be wrong.
If you take Rshares = rating^3/1000^2 and plug in the numbers for middle ratings of each league, 550, 1450, 2350, 3250, 4150 for bronze to champion you get the Avg Shares/Win in the table. This is then assuming 100% ECR and no bonuses and no starter cards.
I am also wondering if the 100 chests data is wrong, jumps a lot from bronze to silver (a factor of almost 10), silver to gold increases only for about 15% and then even decreases when going from gold to diamond...