At least MS backed down from calling their smartphone platform "Windows Phone 7 Series" (http://gizmodo.com/5471805/windows-phone-7-series-everything...). What the heck does the word "Series" even communicate in that name?
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.