Can someone explain part of this wiring diagram to me

In the US, home electrical power is actually delivered in the form of 240 VAC center-tapped, with the center the neutral, and 2 hot lines at 120 VAC each, but 180 degrees out of phase. The current in the neutral will depend on the load balance of the various power circuits connected to the two hot lines. There is nothing preventing all of the loads on one hot line being active, while none of the loads to the other hot line being connected. In this case, the neutral current will equal the hot current. For instance,

^{simulate this circuit – Schematic created using CircuitLab}

Assuming the North American electrical system, the only error that the electrician could make that would cause that symptom would be to swap the GND and HOT wires. Swapping hot and neutral would not do it.

That's a very serious error- and quite negligent in not checking for it, if that indeed is the problem. It could easily cause someone to perish.

If it is the problem you will be able to measure 120V between what you call the third pin on the two outlets.

It's also possible, but unlikely, that a problem with the PC could cause this symptom.

Edit: as others have pointed out, the PC functions so it can't be a simple swap, in fact no single error can cause these described symptoms.

If the hot is connected to both gnd and where it should go.. For example if the hot shorted to the conduit and the conduit gnd was broken.

Looking forward to a follow up on what this was...