Well, hard to say, but there are no mysterious forces or phenomena going on here - it's simple thermodynamics - so for whatever reasons, your shorter/colder ride didn't get the bottle as cold as your longer/warmer ride. Exactly why is hard to say, but probably a combo of different starting bottle temps, length of ride, effect of head/tailwinds (tail would reduce the apparent wind on the bottle and keep it warmer longer), crosswind angle and leg shielding, variations in ambient with terrain, sun intensity/angle-to-bottle, how often you used the bottle, etc.

Significant supercooling isn't possible in this scenario.

- Mark