Not personally sure myself. From my research of that event they only did 60% sig because they were certain a major outbreak would happen. A big outbreak did happen, but fell quite short of verifying. They did want to do day 2 high for 4/27 but questions about the morning wave prevented that. As we know now, that morning wave (really both) didn't.
4/27 probably would have verified 60% with room to spare in all honesty.