Cyrano, I think they're trying to find the optimal crowd flow between the 2 Parks on low-to-mid crowd level days.
Just from watching it while we were there a couple of weeks ago, I can see where they might be going.
It doesn't make good fiscal sense to open both Parks for EE when it's not that busy and I'll bet they were seeing all of the on-site guests heading straight to DA when it was open, followed by all of the off-siters doing the same thing when US opened at 9am.
Big bottleneck
I'll bet their gate entry for IOA was way down, with a lot of people getting paid standing around while everyone was already in or heading to DA. (probably backed up the HE line around 10am heading to IOA too)
Makes sense to see how it works with Early Entry at IOA for onsite guests at 8am, since a lot of those onsiters are going to stay in Hogsmeade, ride FJ, have a butterbeer or hit Olivanders - even out the Harry Potter flow.....
But with Hogwartt's Express starting at 8:30am to carry on-siters over to DA, it still gets those who want to get to Daigon Alley (and Gringotts) ahead of the 9am Universal Studios gate opening crowd.
Now they have guests in US and IOA and flowing in both directions.
We did 6 EE mornings 2 weeks ago when they started this and each one was a little different. Different in what time they opened the gates at US, or when they fired up HE. Looking back on it, it was straight test mode.
And all of it worked fine except the morning HE was down until 9am, really putting us onsiters behind the 9am gate entry at US. Gringotts was filling up.