Ah I see, your misunderstanding turns out to be extremely simple. What you must grasp, and I know this seems too simple to be true, but it's a fact...Each stop of dynamic range for each photocell contains one and only one bit of information!
Therefore for the 14th and dimmest stop, all we need to represent its complete state are the values 1 and 0. Any values beyond those are beyond the precision of the photocell and will just consist of random noise from brownian motion etc. There is no lost analog information whatsoever.
It's also true for every other stop, each bit will remain in base 2, either on or off, and the fact there are more bits to the right is no different than there being other bits to the left. The collection of all these bits that are significant and vary according to the signal being measured sums up to your precision, the number of those bits being your dynamic range. But each stop of that dynamic range is fully represented by its corresponding bit, and no more are needed or could be useful given the limitations of the sensor.
If you could somehow "see more" detail down at that dimness level 14EV below clipping, see more than simply a choice between pure black vs. infinitesimally lighter black, then...you'd have more than 14 stops of dynamic range in your photocell!
Obvious once you get it but hard to get there often!
My point wasn't that the 5D3 or D800 would be "weak" per se, I'm sure that on any sensible criteria, they are superior to pretty much anything. My point was more about the fact that after a point, any extra DR will be practically unusable, and thus, using the calculated (or even measured) maximum DR as a criteria in comparing sensors or cameras is misleading at best. Now, I'm not sure what the practical maximum usable DR with 14 bits is; I picked 10 stops since at that level the quantization isn't yet too harsh in the bottom end (IMO).
Example: Let's say D800 has 14 stops and 5D3 12 stops. Using mild simplification, the D800 can represent the entire 14-bit range of [0,16383] and 5D3 only the range of [4,16383]. I'd assume the D800 now gets a significantly higher score in "objective" rating for DR, since after all, it has those two extra stops. But those extra stops are represented by values 0, 1, 2 and 3. Hardly a significant advantage, I'd say. Nor is it reason to moan that Canon have done nothing in 3,5 years since the 5D2 also had 12 stops.
Sure, dithering in the ADC may help with the posterization effects of the bottom stops, but the resultant pixel data will still be linear 14-bit integers. The bottom stops will still only have only a few possible brightness levels to represent their data.
And no you wouldnt want to snip off any of those bits of dynamic range and the fact they use 14 bit ADCs for 14 bits of information is great. If you used 10bit ADCs your sensor would only have 10 usable stops of dynamic range no matter what you did subsequently. Dithering prevents quantization effects from harming the delivered quality at no loss of dynamic range. You will get all 14 bits and 14 EV's and they will all be equally useful and good.
And my main concern with the reported figures as I said isn't the dynamic range of the sensor or ADCs, it's the narrow 16 bit space the DSP is done within. If that is an accurate report.