The stats I looked at (GoSquared) are collected from the web. They reflect the proportion of users that visit relevant websites when using a particular OS. So things like downgrades are easily traceable. These stats approximate the current, synchrony distribution, they do not accumulate over time. It's certainly far from perfect statistics and it's not very robust, but it gives us a fairly good idea about what is going on.
P.S. Yosemite downgrades were clearly seen on the graph brw. Shortly after the release, there was surge of adoption which quickly followed by a sharp drop. That were people revering back
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This depends on how they do their stats and data collection. A reasonable algorithm would report your IP as snow leopard. At the same time, I don't think that they go through the hassle of tracking details like that. For the big picture, this is irrelevant because users like you are probably do rate that it doesn't matter if they are counted wrong.