When was this? Which NV cards outsold AMD by a ratio of 2:1 when the AMD card provided better performance?
When AMD had a clearly better card, (X800 vs FX5800), AMd took over as market leader. Since then it is hard to say whther AMD had a clearly better product stack top to bottom. E.g. Fermi was late and used a lot of power but it still had the performance including in the latest DX etc., and the lower Fermi models were decent. Plus there is always a lag between product quality andf changes in percieved market and value. The 500 series quickly followed on and Fermi's teething issues were solved. AMD have produced plenty of other good cards, but they sometiems had issues like the 290 heatsink, were around at the same time Nvidia had decent products, or the line up top to bottom wasn't a clear winner. Most market share is at he lower end and so it doesn't matter so miuch if AMD create a good Fury card if the bottom end is taken up with old cards renamed that are missing features or have outdated video codecs etc.
then there is the fact that AMD supporters normally go by value for money, which ignores a lot of other factors people care about. For example, a top NV card may well be 20% more expensive for 10% more performance, but if you want that performance it is worth paying, nothing is ever linear like that.