I bet it depends on the local market. I was at Redmond over the summer and I'd guess the market share was 50% WP, 25% iOS, 25% Android. Some parts of the town have higher WP usage, others less.
Well this is why I'm not surprised that WP is growing in share. For my friends it seems to be if they want to spend money on a "quality" phone they go for apple, if they want a cheap phone then WP. The only people I know with high-end androids are programmers and SEOs.