We’re off time in a predicable pattern
Science lite version
Real science original article
(I read the lite version)
on beat = drumsticks with no hinges.
off beat = drumsticks with one or more hinges.
I think what they figured out is that randomly changing lengths of beats is more noticeable than being a little off but consistently off. I noticed this in recordings of my band… when people get a little off the beat they aren’t randomly a little in front then a little behind; they are either in front OR behind. The time interval stays close to the ideal. And further, the whole band deviates slightly in tempo throughout, and not just when it is making a tempo change. And that helps to explain why randomizers or humanizers just make the playing sound like an inexperienced player. (And I don’t mean to imply that my band is any good!)
That’s a good synopsis and makes sense.
Rubato, the musical symbol for “relaxed” timing means thief and where you steal a little time in music then it needs putting back, or correcting later within the bounds of the overall time of the piece.
Some practising drummers know when they have their timing excercises right when they can’t hear the metronome.
This is because you move “out” with the metronome so that you can hear it over drums and even practise pads.
Bands do this as well. If their is an instrument masking what they need to hear they will move their timing very fractionally away from that instrument usually without even knowing that they’re doing it.
Excercise. Get a metronome. Then get something to tap that’s slightly louder; pencil on a table for instance.
Set metronome speed and play along just tapping with each metronome beat.
Now, tighten that up and try to make the metronome disappear. ie: so you can’t hear it.
That’s right. The louder a band gets the more the timing is shot because you have to move out of it’s way to hear yourself over that guitarist. Then a second later you will either turn up or play louder and everyone turns round and blames the drummer. 
It also gives a good disciplined band it’s character. They play with the band and not with the timing.