How can British people understand American slang, but vice-versa no?

[Mark]

Generally speaking, Americans cannot understand British slang. But most British people can understand American slang, quite well in fact. Does Hollywood have anything to do with this? We get very few British TV shows here, but in British TV, at least half the programs are American. And the movie market in UK is composed of at least 80% American movies. Then comes Yahoo, Microsoft, Facebook, etc...
Is our media and Hollywood responsible for the spread and understandability of our talk?