During secondary injection the breaker is only indicating half of the injected current.
NW Frame Breakers
FFTK testing a NW breaker.
This will happen when a W frame (standard width) breaker is being tested as a Y frame (double-wide) breaker. The FFTK defaults to testing as a Y frame.
To get a good test, on the setup screen where you select breaker type, choose NW40b or test as an IEC breaker. This will force it to recognize it as a W frame and test it correctly.