A lot of money yes, but they released it at 8 PM on Thursday and had the advantage of an extra day with the memorial day weekend.
And by the way, the $400 million was the WORLD WIDE take. It only made $156 mil in the states, and only $142 of it was Friday thru Sunday. So compare: Spidey manke $151 million Fri-Sunday, and Pirates makes $156 million from 8 pm Thursday evening to Monday night.
I think the biggest surprise is despite the success of Pirates that Shrek still make 70 Million in it's second week.