Acording to this article https://fstoppers.com/gear/sony-announces-a7r-iii-solid-improvements-built-around-previous-generations-424-201012 the Sony A7r iii uses the exact same sensor as it’s predecessor. Given that how can the A7r iii get .8 stops more dynamic range than A7r ii as tested by dxomark?
From the article linked:
advances in circuitry design and image processing mean that the new camera can output all 15 stops of dynamic range in the lower ISOs rather than the a7R II’s 14 stops of output.
Dynamic-range is a ratio, so by improving the circuitry, they can reduce read noise. By having lower read noise, even with the same well-capacity, dynamic-range is greater.
Keep in mind that the sensor may be the same in most of its specifications, it may also be different in some ways or it could be manufactured with an improved process which reduces the amount of stray electrons.
Can we start by saying it is not EXACTLY the same? Same generation - yes, but they did make changes. For example they improoved and thanged the autofocus part on the sensor.
It is likely they did some other minor optimizations based on the same sensor design.
Please realize that 0.8 stops is not a lot - they have more than 15 in the old A7RII.
Heck, they even say so in the article you linked. Did you not bother to read it at all?