Really, I don't think they sent invitations according to clusters... It would be unfair against specific industry/regions clusters right? At least because they would give to those less time to find an interview slot in hub cities...
Rather, I would focus on Dee's statement "No secret patterns as to which go out on the 17th vs. the 24th". Last years she used to mention specific splits or distributions of invitations, this year she didn't. I like to take this as a sign of a 50/50 distribution...
And by the way guys, the number of invitations that go out on a specific day DOES NOT INFLUENCE your odds of being interviewed. Your profile has strengths or weaknesses with no regard to the invitation bucket you belong to. The probability stuff can be applied to the applicant group as a whole, not to individuals. So, my suggestion is enjoy life!!!