Ambient air temp when the bottle is cooling probably also plays a role, more or less shrink before it “freezes”

Yeah I agree with that. Also maybe inconsistency with the plastic batch.

But also milk jugs are blown directly from pellets, no threaded blank. Water bottles and pop bottles go from threaded blanks though.

I don’t remember seeing washing but I guess that would be on the filling side, (jugs are made on one side of the plant then go through a wall to the clean side) I don’t think I’ve seen that for milk.