We go to WDW many times a year, and they OFTEN open the parks earlier than the posted time...even on very slow days. I think it has to do with how many people are lined up and crowding the space at the tapstyles...too many people in the limited space between the tapstyles and the actual entrance, and they will open the park early (or if it's pouring down rain...they will open early) Our experience is DHS often opens 15-20 minutes early...Epcot 15 minutes early...MK 10 minutes early. AK is the least likely to open early, in our experience, though it has happened occasionally...but more in the 5 minutes early range.
Now there is a drawback to the early-opening...they also often have parts of the park still roped off, inside the park. Like MK may have rope across the entrances to Fantasyland and across the bridge that gets you to Adventureland, etc. And if they have the ropes up inside, then they generally do not drop that rope until the actual 9:00 opening time. So you get through the tapstyles and into the park, but you're still waiting for 9:00 AM to get on a ride.
At Epcot, when they open early, the rides are usually going. We have been on Soarin' at 8:50 AM on a day the park opened at 9:00 officially. And RnR or ToT at 8:45 AM. But have also gotten to RnR at 8:45 and were told the ride would not go until 9:00 AM opening time. So first on the ride, but not early.
