Hi Dan,
Not sure if I could give you a physiologically based answer, but you might be on the right track. For myself at least I think that timing is a function of my internal clock ie: heartbeat that even though we don't have awareness of it, it influences the pace of everything we do. Musicians train with a metronome for that very reason, but I tend to change the tempo of a piece depending on if I am trying to sing along or not. I might speed the count up a bit to keep within the limits of my vocal range and ability to hold the length of a note. Where if I'm just playing the instrumental, I stay pretty close to tempo as written.
When playing in a group however, as a guitarist not pulling lead, it is important to keep tempo with the percussionist as everybody else is depending on you to set the beat and timing for chord changes. Vary too much and everything gets upside down pretty quick. Drummers do the same thing from time to time and within seconds everybody is scrambling to get it all together.... that's usually when the rehearsal screeches to a halt and it goes back to "take it from the top one more time".
Take Care;
Doug
"what is this quintessence of dust?" - Shakespeare