Moving the sweet spot

I was just walking behind my sweet spot chair, and I had noticed that I can hear more of the high frequency, just before I arrive in the center of the listening position, then I move back and forth to pinpoint the place. Then I move my chair at that new spot, one foot farther back, one foot to the right, the chair pointing between the middle of the speakers. So, It seems that if I am sitting at the sweet spot, there is a high frequecy cancellation. My room is an open space, and my left ear is not as good as I was young.