I don´t know much about how works ratings here but it´s not very similiar to the system of the federation international of chess (Fide) based on the method of the mathematician Arpad Elo.

I think the rating initial (provisional but initial) in brainking is established after of the fist game finished using the rules of the system of US chess federation [http://www.sizes.com/sports/chess_ratings.htm]



This system works in this way
rating initial = rating opponent + {400(win), 0(draw), -400(lose)}

However Fide system is different :
rating initial = rating opponent + {12.5(win), 0(draw), -677(lose)}

The variation of rating in the second and next games works different too.

This system applied here favours to the players win his first game. If you win the first game against an opponent of 1900 then you will have 2300 as an initial rating and you can keep this rating playing against weak players