Because all days are not the same.Some days you are more focused than others or your opponents are less focused than others.The fact that you reached a rating doesn't mean you do deserve it every day and every minute.
Why is this happening to all chess players??
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People seem to lose track of the fact that their "rating" is a "performance rating." It is only as good as your last dozen or so games. If you hit a hot streak, it goes up. If you have a string of lame games, 
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It`s easy to lose more than 100 points in live chess when you are drunk, distracted or when it`s just not your day.
As you gain in strength you'll be playing stronger players, so you will eventually hit a peak and then give up some of the gained points to your new stronger opponents. With study and practice against stronger players you'll get better, surpass your old peak and reach a new one, then go through the process again.
Hello everyone!! I have question to all chess players,,why is it that when you reach your highest rating here in chess.com,,it doesn't stay in that rating so close,,like mine,,my highest rating in blitz live chess is 1403 then now 1319..Can anyone can explain this??