Anyone have an EGR P400 error yet the EGR is fine?

I've had this problem too many times for it to be a unique incident. The eng light comes on, or I fail a smog because of an EGR failure aka OBD2 code P400.
Yes the EGR valve is fine, yes the passages are clear, yes I can watch the valve operate normally right before my eyes, yet still the computer pops an error.

I guess first question is how the #^ck does the computer even know wth the EGR is doing in the first place? There's no sensors on it so wth? I think this is my whole problem, whatever way the computer thinks or reads it as is in error. All I can think of is it's looking for some change in the O2 sensors when the EGR is supposed to be actuated, but in what way is that reliable or even possible? Maybe the reason I have so many cars with this same error is because I drive in a manner that doesn't give the correct O2 reading at the exact moment. I usually accelerate faster than most, as in 99.9% faster than everyone else does at stop lights and I'm always full throttle getting on the freeway and I drive ~80-90mph. I can't imagine that matters, but something is tripping the EGR error...
Cars w/ error: '99 CLK320, '98 ML320, "00 ML55, and this past year it's been a '99 Subaru Legacy.
Also, the CLK and Legacy take forever to get a reading on the EGR, but of course when it does it's fail. By that I mean from the last time the computer was reset it runs checks on things and even after two or more months and hundreds of miles the car still reads that the EGR check is not completed. So again, what determines a good vs bad egr? Nobody can tell me how the computer gets this bad input or determines it as bad so I can fix the problem!
And again; yes the EGR system is working correctly, 100% from ex passage to intake and yes it comes on at cruise and deceleration and closes under load. Every one of the cars I listed had perfectly functioning EGR systems, yet somehow fail..

Thank you...

Just noting that this is still a problem and now I'm adding two more cars to the list, my '04 C320, and '99 Saturn SC2. Again, these and the other four ALL have the exact same problem, P400 error code yet there is nothing wrong and the EGR system, it is functioning perfectly! So we have six cars, and three different makes, yet I'm the only one with this problem??? Having a hard time believing that... Might I ad that the ONLY car I have/had that does not have this problem is too old to have a computer or EGR valve. So every single car I've owned in the past 10+ years have given me this exact same problem, but nobody else in the world??? Some are never driven by me so the only way I'm a factor is my name on the title. I've asked smog techs, and people who've been mechanics their entire lives, everyone I can think of. It seems true that I am the only person in the world with this problem... Even internet searches of this problem only turn up my old posts, nobody elses. That's billions of people and I'm the only one?
I'll ask my basic question again; does anyone know how/why the computer trips a P400 error so I can fix the root of this problem? Obviously something is causing this and I'm tired of selling cars after two years because they won't pass this simple test!
Thank you....