I understand a clamp meter works by measuring the magnetic field generated by current flowing through a wire.
It's well known that clamping 2 wires carrying the same current in opposite directions will show a null reading because one magnetic field is cancelling the other.
What I find surprising though is that if the second wire carrying the opposite current is placed immediately outside the clamp, it has no measurable effect. I was expecting its magnetic field to couple with the clamp to some degree but that seems not to be the case. Why?
Imagine that the return conductor for the red-wire (the blue-wire in your picture) is a million miles away. You would probably agree that it has negligible effect.
Now as the blue return wire is brought closer, the localized magnetic field it produces helps the red wire's magnetic field (on the left) but it also produces an opposing magnetic field passing through the "ring" on the right.
The two opposing fields always cancel to produce the same net effect: -
The magnetic field from the red-wire not drawn for reasons of clarity.