in the FAQs the following is posted:
Players are rated using the following formula:
New Rating = Old Rating + K * (Score - Win Expectancy)
K is a constant (32 for 0-2099, 24 for 2100-2399, 16 for 2400 and above)
Score is 1 for a win, 0.5 for a draw and 0 for a loss.
The Win Expectancy is calculated using the following formula :
Win Expectancy = 1 / (10^((OpponentRating-YourRating)/400)+1)
So win expectancy is >0 and <1 and k is >0 so together this implies that newscore>oldscore for a win.
I don't see therefore how score can decrease for a win...