The party isn't at 6:00; that's just a placeholder time since the party begins one hour prior to Wishes, and the timing for Wishes varies. Here is some of the "fine print" from the confirmation email I received:
"Event Start time will be 1 hour before Fireworks Show.
Fireworks are subject to change and not guaranteed. Guest should reconfirm Fireworks schedule at least 2 weeks prior to arrival and again upon arrival at Park."