From one perspective it makes total sense that Nokia would introduce the 925. Many complained that the 920 was bulky and heavy. The phone might be totally fine once you're used to it, but first impressions matter. Secondly, it's entirely possible that Nokia is privy to data that shows consumers dislike colorful phones. I can see it being quite polarizing, especially since Apple continues to define the aesthetic. Black, white and aluminum for everybody. Some day Apple will introduce colorful phones and everyone will soil their pants in excitement. Suddenly a yellow phone will be every bit as professional as as black one.
Another problem for Nokia is that they aren't in a leadership position. Samsung and Apple are the default brands so things are a lot easier for them. Everyone else has to work a lot harder. Nokia in particular was put in a difficult situation in that they introduced their device early in this wave of phones, their thunder being stolen by the advent of the Blackberry Z10, HTC One and Samsung Galaxy S4. So the Lumia 928 and 925 revitalizes the line.
Now with that in mind, there's something to be said for instilling good will in the minds of owners of existing devices. People don't like to feel like they're being left behind for the next big thing, especially while they're in possession of a device that still feels new. I think that one of Apple's strong points is that they've got a very stable update cycle. They don't start teasing new devices on a few months in to the current life cycle. You can't control the press, but at least there's the confidence that Apple isn't going to leave you high and dry. Granted, the 925/928 seem to be functionality identical to the 920, but perception is what counts.
Part of that perception goes back to brand image. The 920 was visually distinctive. The 925, while attractive, is not. Whether the 925 ever ends up on store shelves remains to be seen. I'm convinced that one of the big problems with the 920 was that stores put no effort into marketing them, assuming they even carried the phones at all, which many retailers did not. I'm not sure the 925 will change that, and it's certainly not going to jump off the display like the 920 would have.