Happens all the time. Planet alignment, position of the stars, movement of the Sun and Moon, or maybe my ears ringing differently at different times. LOL. Could be anything. Even standing in a different location from the speaker.
Just trust in the law of he averages, that it usually sounds right set there. And don't become a fanatic knob twister. Had guitarist that every time they'd turn on a pedal, have to bend down and move that knob again, almost every song. Drove me crazy.
I come from the school of "set it and leave it". If you're always fiddling with knobs maybe your playing suffers so much as to make more difference. At least that's what I noticed from some players.