Sure power supplies provide accurate power ... till they can't. Unlike peak power that starts to degrade from new, the voltages will hold as long as the PS can manage it.
And again, the ATX PSU doesn't really matter here, the M/B VRM does. Even if the ATX PSU has a failure mode where it delivers 10V instead of 12V — you can still make 1.05V out of 10V. Decoupling is spread all the way too, so short peaks don't explain a lot either.
Lastly, it's of course possible the VRM is misdesigned. But this issue is seen across mainboards from at least 3 vendors… what is the likelihood of all of them getting it wrong in a very similar way? The primary way for that to happen is if they are all implementing Intel's reference design, in which case it's Intel's fault again anyway and we've looped back to the origin of the discussion…