On HN, you could give people the benefit of doubt and assume that market-share growth wouldn't be brought up unless the numbers were large enough. I would refrain from quoting that figure if it went from say 10,000 units to 20,000.

Last quarter, iPhone sales were 31 million devices and WP8 sales were 7.4 million devices. If WP8 is selling one device for every 4 iPhones, that is indeed a decent result relative to where they started off from.