I really shouldn't mention this stuff, because on one side there's that great big blower manufacturer, and on the other side there's jus' li'l ol' me... and the laws of physics.
I don't know if they don't understand what's really going on, or (more likely) they realize that most people visiting are looky-loos (not buyers or owners), and a complex answer will just tune them out.
The answer given, "final compression ratio", stating flatly that's it's a ratio, is a poor choice of phrase. Why don't they say, as qparker said "the equivalent"? because that just precipitates another question. They're not in the question business, it doesn't pay well.
The pressure does go up, but nothing like as fast or high as shown. However: the results (although the math is bad) are useful in that they're all conservative. A really complex answer will not only give the new pressure equivalent, but begin with the original pressure.
???
Yes, the original pressure was perhaps 1 psi of vacuum (2" Hg) for a hot V8, so the increase is larger than it appears (10 psi is not 10 higher, but 11 higher). However, for an under-carbureted motor such as a 250 with perhaps 2-4 psi @ WOT 10 psi is much higher (12-14 higher), and would give results closer to their math.
Clint Gray did the research for this, I just worked out the language: http://victorylibrary.com/mopar/blower-engine5.htm#ECR

BTW: they finally put that CFM calculator up... after mine (and perhaps others as well) was pointed out to them.