If the best team won the cup every year, then every series would be a sweep (or we wouldn't need a seven game series). Sometimes a team who isn't the 'best' manages to win it.
Two of the best examples of this (though not from the NHL) would be the US Olympic team of 1980 and the Pittsburgh Pirates World Series victory in 1960.
Two of the best examples of this (though not from the NHL) would be the US Olympic team of 1980 and the Pittsburgh Pirates World Series victory in 1960.