It looks like sloppy book-keeping rather than systemic cheating from the bit I've read. However, as a flagship club, it is embarrassing for the sport and Wigan should do better.
One of the issues with a salary cap is that it leads to a competition over who can cheat it the best (amongst those who can technically afford to breach the cap) - and then of course you need to spend loads on accountants to forensically check what all the clubs are up to.
Very difficult to enforce and, to be fair, for clubs to administer the compliance.