There's very little parity between the best teams in European leagues and the worst. In the NFL, a hugely parity driven league, if New England loses to St. Louis in the regular season, it's surprising but not a historically significant upset. If Queen's Park Rangers (worst team in the league) beat Manchester United, it's seen as a monumental victory for QPR. This is largely because, the gulf in class and player salary an between the two teams in so vast.
(Although, QPR has a larger than normal salary for a crappy team.) It would take Manchester United drawing several games to vastly inferior teams, and losing to a few teams around its talent level.
As a counterpoint, MLS is usually a much less predictable league, in part because of a salary cap and playoff system that encourages parity.
This post was edited on 3/1 at 10:43 am