I don't mean to be argumentative, but it is a matter of FACT that buses start running earlier than 1 hour before park opening times. Disney knows that they offer early ADRs at some theme park restaurants, and they like it when people depend on their buses, and therefore they DO make it possible to get to those ADRs using Disney Transport buses.
You can catch a bus to MK as early as about 6:30 a.m. to get to a 7:00 a.m. ADR. Disney Transport buses generally run about every 20 minutes, but those very early morning buses are more like every 30 minutes.
The last time I was at WDW, we were hoping for some early morning EMH fun at Epcot. I didn't know that in between the time I wrote down the park hours at home and the time of our trip, that Disney had taken morning EMH away from Epcot that day and given it to the then-named MGM. So anyway, the point is that my son and I boarded a bus at POFQ at 7:20 a.m. and got to Epcot plenty early for the 8:00 a.m. EMH, only to find that it really opened at the plain ol' 9:00 a.m. But people were getting there early for their early morning ADRs at Restaurant Akershus. In other words, we got on an Epcot bus 1 hour and 40 minutes before the park opened.