They went with overestimation intentionally.
I'm a local, and from talking to people that work at both places (and I've seen this mentioned elsewhere too, but those in the know say it is pretty accurate):
It seems Universal's attendance has been up significantly since the expansion opened, to the point where they are surpassing park numbers in all but the MK on Disney side. Guest spending is well up, and Universal's hotel occupancy is over 90%. (Higher than Disney's) All the 5 year expansion plans are still on, and actually have been plussed based on Diagon's results.
One thing that really worked out well for Universal was building Transformers and Dispicable Me, which has evened out the crowds a great deal. It wasn't just the Potter area doing well. As well as handling the crowds much better than when the original land opened up.
If this was the truth, it would be widely reported upon, not just people that work there talking.
As for hotel occupancy, everything I see of Cabana Bay, the resort is nearly empty.