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Its easy to see that some people download multiple times, others download it but don't use it.
However, I have downloaded it once at home and that means two users in two different machines (me and my sister), and I also downloaded it once at work, but that means dozens of users in dozens of different machines.
We also have to factor in the Linux distributions, which don't add anything to the download counts, installs from "shareware" DVDs in magazines, etc.
In the end, 25 million downloads doesn't show the real numbers, but gets really close.