Newest HP film rated 12A in UK while PG rating remains in the US

Earlier today, it was revealed to major HP fan websites that the new HP film would recieve a 12A rating in the UK, which is the
equivalent of a PG-13 rating in the US. Yet it was previously announced in the US theatrical trailer for the film that it was rated PG for the following: scary images, some violence, language and mild sensuality. So the question remains, Why is it confirmed to be PG in the US, and recieve a higher rating in the UK? WB has refused to comment on the matter.