So it's worthwhile to make sure we all understand that we're talking about two different things: atrial fibrillation and sinus rhythm are two options in the same bucket: rhythm. You can have PVCs with afib, you can have PVCs with sinus rhythm. You can't simultaneously be in afib and sinus rhythm.
I'm guessing Apple is just looking at rhythm - it would be the most reliable datapoint, as opposed to trying to measure QRS duration or analyzing for presence of P-waves, both of which differ depending on which lead you're looking at. The lead being looked at depends on which two points on the body are used to produce the tracing as well, which is not something Apple can guarantee, so perhaps that's where the explanation lies, but I have no evidence or data to back up that conclusion.
I'm guessing Apple is just looking at rhythm - it would be the most reliable datapoint, as opposed to trying to measure QRS duration or analyzing for presence of P-waves, both of which differ depending on which lead you're looking at. The lead being looked at depends on which two points on the body are used to produce the tracing as well, which is not something Apple can guarantee, so perhaps that's where the explanation lies, but I have no evidence or data to back up that conclusion.