The circuit breaker gfci works similarly to a clamp ammeter. For example, if you had L1, L2 and Neutral and you put a 240 volt, 10 amp load on the system, L1 and L2 would each carry 10 amps.

If you measure L1 or L2, you should read 10 amps on each leg. However, if you clamp over both lines at the same time, the current would read zero because the current's magnetic field in the forward direction cancels the current's magnetic field in the reverse direction.

If you then added a 120 volt, 5 amp load, you would measure 15 amps on one hot leg, 10 amps on the other hot leg and 5 amps on the neutral.

A clamp ammeter around any 2 or 3 wires will measure the net difference between current in both directions.

Putting the meter around all 3 wires should cancel and read zero amps.

As long as the neutral goes through the breaker, the total current in both directions should cancel.

So, a breaker doesn't really measure current and compare to make sure that the current is the same. It works on a cancelling effect.

As long as the current in both directions cancels, the breaker won't trip.