They always board A380s through multiple doors. But they probably had a flight full of Germans - Americans tend to take a lot longer to get themselves sat down than most Europeans do, and spend a lot more time faffing around with their bag and belongings while blocking the aisle.
The Japanese are still the most efficient at boarding though.
There's also the difference of airlines. Airlines that charge extra for checked bags will have more people messing about trying to fit in a carryon, not finding room, etc. And U.S. carriers tend to have some of the worst policies. (in other places LCCs have the same policies but they also tend to have smaller planes)
The Japanese are still the most efficient at boarding though.