to some it may seem like a tautology: if you quit ladder games which you would probably win, your rating becomes inaccurate. i play maybe 50% of all the ladder offers me, because that is about the fraction of maps i find worthwhile.
i find playing against 1v1 rating of 1400 to be suited to my skilllevel, still i get a lot of opponents below 1000, because my rating is in fact 1150. so how about not counting games that were not actually played. like game time < 10s.