Helps to have a weak field.
The field wasn't that deep to begin with, but...
Americans place well in major races because they ignore the monster surges that are laid down by the eventual winners. Stronger runners, who aren't as strong as the eventual winners, chase those moves, blow up, and drop out, leaving the Americans to mop up the minor placings. We saw it today with major players like Stanley Biwott, Desisa and Ritz dropping. Ward demonstrated this in the Olympics. Even Hawkins showed this in the best field ever assembled by finishing 8th in London this year.
The US (and other non-africans) have put down good performances, but there is some self deception going on if the results are taken as indicators of their standing on the world stage.