It's because you play with players that have significantly lower rating. If you win such players it doesn't mean you have improved a lot so you get little points credit. If you lose to such players it comes as a sock and your rating is drastically decreased. The more probable the outcome the less affected your rating will be and vice verca.
every time I win I get only maximum 6 to 7 points credit.
when I loose the game they debit from 10 ro 12 pointsHow does this work