Perhaps you've got the US version?
In the US, they have one GFCI (as they're known there) per socket, so the trip current can be lower, without nuisance tripping. I believe in Australia, one RCD is used for an entire circuit on a 16A breaker, so a higher tripping current is necessary to avoid nuisance tripping.
I'm always weary about modifying test equipment, especially the type used to test a safety critical device such as an RCD. I strongly recommend you buy one with the correct current setting, rather than modifying it.