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Microsoft CEO Satya Nadella admits giving up on Windows Phone and mobile was a mistake
(www.theverge.com)
This is a most excellent place for technology news and articles.
Internally they were two very different things. WP7 was their old windows mobile with a new skin, while WP8 was the actual OS they had been working on for a long time. They felt the same but were very different. I guess they didn't think it was worth the hassle trying to figure out how to handle that update.
I don't know what was the deal with 10 though (I forgot it even existed tbh).