When I saw The Hunger Games I sort of couldn't believe it was PG-13. It isn't that the movie is especially explicit, it's just that the movie isn't afraid of killing children on screen. And there's one bit early on in the actual Hunger Game where a half dozen children are murdered over the course of less than two minutes. It's a bloodbath, and it's very intense. The MPAA usually frowns upon violence towards children in PG-13 movies, and The Hunger Games has that in spades.
But the movie made it through the process intact... in America. The UK has their own weirdness, though. Back when Attack of the Clones came out UK audiences weren't allowed to see a head but during the Obi Wan Kenobi/Jango Fett fight. And now the UK has demanded seven seconds of trims in The Hunger Games in order to give it a 12A rating, instead of a 15.
The seven seconds are not specified, but I'll bet they're during that bloodbath, a scene that will probably become known as "The Cornucopia Scene." Director Gary Ross has really created something specific there, and I wonder how even the most minor edits will impact the mood and intensity of that scene.
EDITED TO ADD: According to the Guardian Lionsgate also digitally altered some scenes that remain in the movie, removing blood from weapons and wounds.