That it was the start of a new mobile platform. It made a little sense at the time. I think the logic was that Windows Mobile 6 to Windows Phone 7 just sounds like a rebranding with a version number increase, whereas "Windows Phone 7 Series" sounds like a reboot, a new lineup.
It's also entirely possible they planned separate editions, not unlike the basic/pro/touch editions of Windows Mobile 6. So when they announced it as Windows Phone 7 Series they has two or three versions in the pipeline but come release the had cancelled all but one so it lost the "Series" moniker.
I'm not sure but I would be surprised if there was no at least slightly logical explanation.
