Around Christmas time, that PG rating could help them a lot, especally when competing against other kids movies.

Having a trawl of the classification website, you can see that although the running time is the same, (114 min), something might have been cut, as the M rating has a very mild rating of drug use and sex.
These are just themes, so it could include someone drinking a beer on screen and not the act of sex, but kissing etc.

The PG only has mild impact for themes, violence and language.

Or, it could well be that they just appealed on the grounds that the "impact" those three sections that pegged it for an M rating are so minor that they are not worth worrying about.

Chances are the movie was initially viewed as being at the upper end of the PG and very very lower end of the M, so got slapped with the M to err on the side of caution for the classification board.