I would argue it is possible for better classification; I don't have a reason why apple chose not to do this (especially if they're capturing the data) other than the algorithms are not as guaranteed as we would like and they don't want to assume that kind of liability. (ECG printouts include the computer's interpretation; a standard way of reading them is to completely ignore whatever the computer says, because it is not infrequently completely wrong. And those are the professional grade machines with 12-leads capturing 10 different points on the body)

Edit; As I thought more about this, I realized that analyzing QRS duration or presence of p-waves can be lead dependent, which can change based on which parts of the body are being used to generate the lead.