Clamp tests are not accurate at all, as the formula used is a DC formula on an AC circuit. They almost always report more power than the amp is producing though, so I'm wondering if the problem you have with them is that they produce too much power over rated or that they're too inexpensive to produce that much over rated?
But either way, when your methodology is flawed any conclusions drawn from those false premises are also, necessarily, flawed.
And keep in mind, Coleman didn't go out and find some new build house with some new board design that had never been tried and tested before by some other manufacturer. There is a lot less diversity in the nuts and bolts of electronics than most people would believe.
Just sayin...