I think Quarqs measure total power but estimate how much is coming from each leg based on where in the pedal stroke the power is coming. I.e. When the left leg is at 10 o clock it’s going to assume that most/all of the power at that point is coming from the left leg, but if you’re better than the average rider at unweighting or pulling up with your right foot then that assumption (and your reported L:R balance) will be a little off. Since it doesn’t actually have a strain gauge on the left and right cranks so knows how much total force is being applied to the spider but can’t tell how much of that force is coming from the left leg pushing down and how much from the right leg pulling up.