EVERY cert test I've ever taken tests not knowledge of the subject
Because that's not their main value.
Hiring managers don't know enough to qualify candidates. So they hire people with certifications. That way, if the employee sucks they just say, "hey, he was properly certified - blame the certifier, not me."
It's CYA, blame-shifting, etc. The ability to deflect blame is quite valuable to people who are not qualified to be in their jobs, so they're willing to pay more to such employees, because such employees are valuable to them.
If the tests get too tough, the candidate pool will dry up, so that's never going to happen. For qualified interviewers, asking the questions isn't hard, so they don't have a use for the certifications.
Another factor is that such middle managers tend to be in the corporate world, as startups cannot afford either the inept middle managers or bad employees. Corporate jobs tend to pay more, so 16% does not seem unlikely at all, in aggregate of the two factors.
For that 16% you probably have to work at a dreary soul-crushing job, but if your interview doesn't consist of smart people asking you tough questions, you probably knew that already. Get busy with the 401(k) allocations so you can live out the end of your boring life in a median retirement community!