Some umpires enforce the balk rules as they are written, others in the spirit of the rules. Umpire "A" might let a kid move his shoulder slightly to see first base, if he doesn't think it is intended to deceive the runner, while Umpire "B" will call it a balk all day.
The problem could be that kids get used to the way a certain set of umpires does stuff then they get a whole new crew in the playoffs.
Didn't Cales get nailed for a balk, under similar circumstances, in the championship game a few years ago?
When the younger brother was pitching travel ball, they would try to check with the umpire ahead of time to see what his feelings were on borderline/judgement stuff like that.