According to the adver the rules are:
To qualify as home-grown, players must have been registered with an English or Welsh professional football club for at least three seasons prior to their 21st birthday.
But they list McCormack as home grown:
Against Burton Albion last Saturday, Di Canio included nine players in his squad that are classed as home-grown. Callum Kennedy, Joe Devera, Matt Ritchie, Raffaele De Vita, Jon Smith, Alan Connell and Alan McCormack all started the game, while Miles Storey and Phil Smith were on the bench.
Wiki have him registered at Preston in 2002