I worry about the demo boards being radically different in terms of layout etc. Even if you're using the same interface and power supply, the PCB may be affecting performance.

Getting full spec performance out of an ADC requires having good layout power supply routing etc.

I would transplant the chips from PCB A to PCB B and vice versa. See if the performance follows the chip or the PCB.

Also check power consumption before / after board swaps. If they are fakes, that would be significantly different.