1. The RIAA standard is not followed by all, and it is not followed to the T world wide. Often the IFPI is used and their certification numbers are all over the place:
http://en.wikipedia.org/wiki/List_of_mu ... ifications
2. The RIAA does not account for all units sold forever, especially if the artist or someone acting on the artist's behalf is not paying for the tracking via Neilson SoundScan, etc. So, anything people have reissued, the bands and artists have sold on the merch tables, on their sites, etc has likely not been reported.
Example

I happen to know for a fact that in foreign markets, product once made under license continues to be manufactured and not paid for. In some markets, it is distributed for free. The black market for Christian music is huge and viewed as necessary. So, since the certifications do not call for units to be paid for (heck, there would be only a handful of artists who could claim that), only shipped by distributors, these albums should be counted. But, then again, there is no way to know, until 70k people show up at a concert and know every word of every song, sporting black market T-shirts, etc. This has happened to Rush, Petra and slews of others, who never knew that they had a fan base, because they never had any documented sales.
3. The whole sales thing is smoke in mirrors. There are many albums certified Platinum, even after 40 to 50k units were returned. The industry has no morals and that certification is massaged to make people money. So, yes it is nice to have that certification for bragging rights, but the numbers are not always legit.
4. Petra knows what they have done, and we all know that in the early days there was no accounting and payments for royalties and such. Bob would not let that statement hang out there if he didn't think it were true. Because the site does not claim who certified them and when, I would say that they are safely telling the truth.