Hope this helps: http://erbtest.wordpress.com/2008/07/30/whats-on-the-wppsi-iii-test/

If that's accurate, it looks like not only is there a time limit on the block design task, but that she likely simply had trouble with 19-20, i.e. she didn't get shorted on questions. That is, unless there's an early termination criterion (miss X questions in a row and stop, etc.).