Even if Disney's filling the rooms at 90% occupancy for the month (about where they max out). There's no guarantee that the off property locations are filling at their rate (and historically, they have been up to 25% lower than Disney's rate in the 'slow' seasons). So you may have a full house onsite, but FAR fewer day guests coming in. That makes a huge difference.
There are also blocks of rooms that are in the hands of resellers (Orbitz, Expedia, Travelocity, Hotels.com, etc. I agree that the resorts may not be as booked up as they seem to be.The WDW website is SUPER wonky right now. It tries to apply an offer anytime you look for rooms, and because there are limited rooms tied to each offer, the availability of rooms appears to be very low. There are still tons of room available. You just need to look for rooms under a "room only" booking. The thread linked below discusses it and how to get around it.
http://www.disboards.com/threads/september-question.3438432/#post-54233259
Dan
I would add that if on property hotels are at 90% occupancy, you can be sure the average number of people that take advantage of EMHs will be roughly the same between busy seasons and September. So, don't expect EMHs to be any different in terms of crowds.
The above truely is the best advise!My best advice: Have a touring plan with your Must-Do's, but be willing to deviate from it and roll with the punches. And unless you are park hopping, avoid the park with Extra Magic Hours.


We have come down in September and first part about of October for several years. It seemed to be busy but not even clear she to summer time or holiday crowds. Weekends are busiest as well as the food and wne festival.We are staying on property for a trip in September. I had read that that is usually a less crowded time to be in the parks. However, it looks like almost all the resorts are booked. When that happens are the parks usually crowded?