But they do a really poor job of it. Anyone who knows anything about studies should see a few holes - User polls are almost never accurate. Each company makes mistakes on their reporting and this can make the ending results much worse. Often the results are predicted based on sales into the distribution channel. But that is not sales to consumers and does not factor in returns.
SPA takes into account only the largest companies.
Microsoft alone get 60 cents on every software dollar in the PC marketplace - yet well under half that number in the Mac market. Usually all these sales get registered as PC sales - which is a double hit. Macs are not getting their volume, and PC titles are getting credit for Mac sales. The SPA makes no effort to accurately track a markets new sales vs. upgrade sales. But the difference to the consumer is dramatic.