Not a huge surprise, and has to do with the differences between the way hardware and software is developed.
Lets say they are shooting for April 15th for the hardware (the watches). That means they have to have them produced and flown into big warehouses in the States prior to that - probably about a week. If they find a last-minute problem with these, they'd better hope it is too small for consumers to notice, because they aren't going to throw them away and make more. Or, even worse, make field modifications to all of them. How would they do that? Hire an army of engineering grads, give them all soldering irons, and start opening boxes in a warehouse? The answer is they can't.
Software, especially these days when everything is electronic delivery with no physical media, is a completely different story. If QA finds a big problem with the software at the last minute, it can be delayed two days for a fix along with regression testing. Yeah, everyone still panics, but there is a solution. Even in the old days when we pressed software CDs, they would be produced a week ahead, and could be scrapped and re-pressed in an emergency.
So, what I'm saying is that software can be changed all the way up to the last hour. Hardware can't. Because of this, releasing them both at the same time when you don't have to doesn't make sense. Plan on the software coming out a week before the hardware. If something unexpected happens, it ships two days later - no big deal. Plan on them coming out at the same time and you give up that buffer because the hardware can't be used at all without the software.
Didn't they do this with iOS 7? I'm fairly certain I remember that coming out a week before the new iPhone release that took advantage of it.
All this said, I am disappointed that they are hedging by saying April for the watch release date. I can't imagine that was their original target. Sure, I'm glad they held it until they could get it right, but like most (and Apple too, quite probably) I was hoping for "early in the year" to mean actually early in the year.
Like February.