...where as this time they gave us a window saying "April 24-May 8" and it appears they're spot on for delivering to us within that timeframe thus far.
I agree that you have a point, but I also think that's a bit of 20/20 hindsight. Ever since the watch was announced last year it was always touted as available in April. Tim never said "April or May" in his announcement. All marketing materials later were refined to announce a launch of 4.24.2015. It was plastered everywhere. The april-may dates were conveniently made at the very last minute and I didn't get a window of 4/24/15-5/8/15 until after I had placed my order. So I'm not sure that it's totally fair to say they are on track with their promises.
So I think the explanation is really launching many more SKUs than they ever did before with a product that they were uncertain which SKU would have the highest demand, therefore playing it safe with the estimated delivery times.
Again, I think you're right to a degree but I also think that's a bit too apologetic. The app clearly had a way to gauge customer favorites for a full two weeks. They surely had some insight as to what would be popular. They hired a former CEO of a major fashion retailer. They should well know how to gauge market interest and ship a product with multiple SKUs.