I do not understand the PG-13 rating, parental guidance is parental guidance.
The difference is how intense the content is when compared to the average PG movie these days. PG-13 movies would have things like a little gore, intense action, stronger language, drug usage, intense scenes, and sexual themes. Let's compare something like say Shrek 2 where as far as they would go would be Donkey freaking out at a pantless Shrek to Transformers 2 when a Deceptacon was trying to make out with Sam with a snake-like tongue. Another good example is how hell is as far as Stuart Little would go or The House Bunny would say "That's ****ing hot!". I can't really imagine the average Marvel film these days with a PG rating since half of what goes on in such films aren't family friendly to begin with.
Video games are arguably guilty of this too sincesome early E rated games deserved the nonexistent E+10 rating these days. Like how Klonoa's grandpa was clearly smoking a pipe in Door to Phantomile and Chain Gun said "Opening up a can of whoop***" in Evolution Worlds. Did I forget to mention the pole dancing meercats in LittleBigPlanet of all things?