Ace uncovers it in a (generally positive) review of The Woman in Black:
The PG-13 rating is misleading. It doesn’t have the typical red flags of an R-rated movie. And I guess you can’t give someone an R for mood and tone and pervasive morbidity.
So they got a PG-13.
But it’s really an “R” movie, as far as kids.
This is a problem with the ratings system. Total Recall has graphic violence, language, and nudity, so it’s R-rated, but let’s face it, it’s cartoonish (fun, but cartoonish) and is going to have next to zero impact on a kid.
On the other hand, this movie doesn’t have those things, and is going to cause nightmares.
The MPAA doesn’t exist to prevent nightmares, but to provide a fig leaf against criticism from the cultural right. As long as it exists, the movie industry can say that it is censored and oppressed by prudes, hence the need for daring, bold films which attack your taboos re: sex and violence.