Physical leads (wires) and "leads" on an EKG are not quite the same thing. Some of the "leads" shown on the EKG are a direct representation of the signal on the wire itself, others are calculated from the signals on the physical wires.
As a result you may get a 12 lead display on the EKG but some of the leads may not be showing a true 12 lead display but are replications of other leads.
