Dunno...one theory (i.e. wild guess) is that the French/CDG consider BUD screening is compliant with their own standards, so no need to re-screen going in that direction whereas OTOH they don't consider the likes of TSA screening ex the States as compliant so you do need to be re-screened before the CDG - BUD flight.....
I'm sure someone will be along with a better/correct answer before long.