As far as I can tell there are three major flaws in the calculation of this figure:
Firstly, it appears to assume a 100% conversion rate between "pirated copies" and "lost sales". As has mentioned repeatedly in other comments, this is impossible to justify.
Secondly, it seems to presuppose that the Average Selling Price that would be achieved in emerging markets like China and Brazil are the same as the current ASP that they get in the Western world where more of the software is purchased legitimately; this too is unsupportable.
Thirdly, they ascribed zero value to the marketing benefit of people "stealing" software in order to determine if they like it and then going on to buy a copy. Repeated studies in the music market have shown that people who download music buy more music and while the situations are not identical it is clear that many people get hold of pirated software to try it and then buy the software for the support that comes with a legitimate copy once they decide that it does the job. Killing illegal copies of software would therefore likely damage sales that they currently make while possibly bringing in some new sales.
Having run software businesses in the past I appreciate that seeing your hard work ripped off can be a serious problem but the BSA spreading mis-information and unsupportable assertions as if they are fact does nothing to make people believe that they are anything other than a bunch of self-serving scaremongerers.