There was a certain element of luck to it all, and a certain element of tactics.
It's not necessarily being disrespectful to your opponents to play a slow and tentative game while you sound out their playstyle and how much of a threat to you they are, whereas against a top opponent who you know inside out it's often best to push hard for an early goal as it puts a lot of pressure on your opponent - doubly so when you're in a knock-out game rather than the group stage.
And if you get a bit of luck with a through ball which gives you a definite goal opportunity in the first 10 minutes that might not have come off 9 times in every 10, it can completely alter the mindset of both teams. Suddenly the winning team can feel infallible whereas the losing team can feel like they are fated to lose. That kind of mindset is contagious, and can lead to one team playing up to their potential and producing more pieces of skill that might not work most of the time, whereas the other team become very insular and eventually reach a point where they just can't string two passes together.
To put it another way: if the US and Germany had played each other 1,000 times, the US would probably have lost 7-1 a good few times. However, once in that 1,000 games the US would probably win 7-1 also. As they say, goals change games. You can't read too much into a 7-1 regarding the strength of both teams, there's more to football than just skill.