Makes sense... sort of...
Well, Nokia does have the WP market all to itself - HTC and Samsung don't seem terribly bothered by it for obvious reasons.
This is a sensible piece of positioning - WP works better on big screens, and it pre-empts the next wave of phablet things for people who can afford a £500/£40pcm device.
In the context of the wider range, it all starts too look odd though. There's the premium 1020, with silly camera. Then the 925 which is a special/tweaked version of the 920, which is near-as-damnit the same as the 820. These are the upper mid-market corporate devices. Then there's the 520, 620, 625, and 720 in the budget section, although the 625 is the biggest of all.
I wonder if another reason for this huge range is down to retail space hogging? Back when the 800 and 900 were launched, you could find WP devices in a phone shop if you looked for them - usually tucked away in a corner. By having 8 devices, along with the couple of HTCs, they're going to be far more prominent in the store.