A lot of magazines’ guaranteed circulations are lies. As a former magazine circulation manager that doesn’t surprise.
It’s pretty easy for magazine publishers to lie. The auditors have loosened the rules and are pretty cozy with the companies being audited (sound familiar?).
Even so, magazines can’t even tell you how many of their “readers” actually looked at the magazine, let alone any of the ads.
The article linked above goes on to say:

Why do we all (advertisers included) winkingly put up with the Big Lie? “Because everyone does it,” says one editor. “Advertisers are just used to paying more per reader than they or the magazines are willing to admit.”

Please remind me of why they’re unwilling to buy Internet advertising, where every “page view” represents an actual page viewed?