This was the email response I received from them this evening:
Thanks for contacting us! We stand behind our numbers and in our experience they are accurate within one index point 90% of the time. Historically, the last week of the summer is a slow week and last year it was extremely slow. We saw some of the lowest wait times of the year in the last week of August. Yes,LaborDay falls later than usual in September but I don't see why that would change anything. September is always very slow and we expect it to be that way this year as well.
It is worth noting that although the calendar uses historical data it is not simply a restatement of what we saw last year with the dates shifted to account for holidays. It is a complex statistical model that measures hundreds of factors then makes the best prediction possible.
Disneyland is always crowded but the wait times are expected to be low.
So, I don't know. The main reason we want to go is to see paint the night and the fireworks. Worth it, or wait a couple weeks until our next opportunity to go and hope it's not too hot then?