Player rating for tournaments across all installments of Heroes.

You are mistaken :) Count more carefully... 100-80=20.

Added 3 minutes later
Rounding is done according to the rules of mathematics... But it's not essential. I did the calculation in Excel - it takes into account all the digits after the decimal point - and the result can be presented in any form with any number of significant digits. Although, to be honest, one digit after the decimal point is quite enough for the rating...