Sorry for the delay chaps - been rather busy - from what I remember of (hazily) learning law, the arguments were about excuse vs justification.

An excuse did just that - excused the action. It offered a valid (in the mind of the defendant) reason for why the action had occurred and why the defendant should not be held responsible for it.

A justification justified why the action had occurred but made no attempt to excuse it. As in 'pleading guilty with justification"

Perhaps in transport to Ontario that justification has got translated into explanation?