Can't tell, unless you tell us your full specs or the Power Supplies and Video Card
Brands at least.
Reply with these information:
- PSU1 and PSU2 Brand and Model
- GPU Brands and Model, which of them are connected to PSU1/PSU2
- Each GPU's overclocking settings
Then, others/I may try find out the reason why.
Here's a hard guess: Some GPUs of the same model can be more efficient (
hardware quality) than the others,
the chance is higher if their Brands are assorted (
different manufacturers).