I think the difference is stress.
These past few days I have been sounding progressively worse. My rhythm is nonexistent, my strumming patterns make no sense, and I can't even fret a simple Dmaj without looking at the fretboard. It wasn't until today that I realized this streak of mediocrity is being caused by bad habits. If I don't consciously pay attention to my posture, I arch my back into the guitar (so I elbow myself in the chest when reaching for higher frets), fret the strings with the flats of my fingers, and pinch the center of the pick between my thumb and forefinger instead of pressing the top edge against the side of my forefinger with my thumb. All very, very bad things. If I had a teacher he or she would occasionally stop me and say, "No! Wrong! Stop doing that!" but all I've got to keep me in order is the occasional reality check.
Going to spend some time going back to basics.