How do they calculate this? Oh, it comes from the "Business Software Alliance", a lobby of software companies.
From wiki:
These estimates have been criticized as being exaggerated and many flaws of the methodology have been pointed out; some of the figures seem to be guesses rather than solid data, and some data may not be representative. The calculation of the losses, in particular, assumes that each piece of copied software represents a direct loss of sale for software companies, a very contested assumption.[6] The study's assumptions have been described as being unworthy of a first year student of statistics.[7] InAustralia, a draft government report has described these statistics as a "self-serving hyperbole", "unverified and epistemologically unreliable".[8]
Anyway, as WereKitten said, these numbers by themselves are meaningless even if true. A lot of the pirating has no consequence to the companies' bottom line.