It's pretty straight-forward - explaining it however may make it seem complicated.

Assuming for the sake of this discussion that:
- toe-on on the front is zero
- steering wheel centred
- distance separating the 2 front wheels = distance separating the 2 back wheels
- frame and suspension components are all in perfect condition

Then: if you were able to place a straight edge from the front rim edge to the back rim edge on either side, they should align perfectly. That straight edge would also be perfectly parallel to the centre line of the car.

With zero toe, it would always be possible to make the two front wheels perfectly parallel to the centre line of the car, simply by adjusting the steering wheel one way or another.

With the front wheels parallel to the centre line of the car, if the rear wheels were not perfectly parallel with the centre line of the car, there's no adjustment in the front that could correct this. And there are no rear wheel adjustments available.

Even if the rear wheels are perfectly parallel with the centre line of the car, if the rear wheels aren't also exactly behind the front wheels (the straight edge test above), then again, there's no adjustment in the front that could correct this (any attempt to correct with toe or steering wheel adjustments here would make the fronts not parallel with the car's centreline), and there are no rear wheel adjustments available. Theoretically, if the rear wheels were perfectly parallel to the car centreline, but were slightly offset left-right from the fronts, then wheel shims could move them out to align them, but nothing could move them in. Practically speaking, this would only occur if there were suspension issues (parts replacement) or if the car's frame had been bent in an accident.

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David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near Toronto