Dynamic range is not determined by the sensor’s pixel count alone. Even if the A7R III is based on the same sensor design, improvements in surrounding electronics and processing can increase measured dynamic range.

The key factor is read noise: dynamic range is the ratio between the sensor’s maximum signal and its noise floor. If Sony improved the camera’s circuitry, analog-to-digital path, or processing so that read noise is lower, the usable dynamic range can go up even with similar full-well capacity.

Also, “same sensor” often means the same basic sensor architecture, not literally every detail being unchanged. There may be small revisions, manufacturing improvements, or on-sensor changes such as autofocus-related updates.

So the most likely explanation is that the A7R III uses a very similar sensor platform but with refinements in electronics and implementation that reduce noise, especially at low ISO. A gain of 0.8 stops is plausible from that kind of optimization and is a relatively modest improvement, not a completely different class of sensor.