I think you may have been referencing my example, in which case, the special training is the reason the NPC has something a player doesn't.Same here. And same with the answer upthread about "secret technique". If NPC warriors from the Sheng empire can cast fireball once per day, and I am told that I should enroll in a Sheng garrison for a few years to learn it... DM, be prepared to have the campaign about the group passing as a recruit and knocking at the nearest garrison to learn it (certainly quicker than the average Joe) before resuming standard adventuring activity. At best, have a subclass of Sheng warriors for the fighter if something happen... "my next character will be a Sheng deserter joining the good side. Can I cast fireball?" Heroes being heroes means that they should achieve what they strive to do. It can include, and in my experience often include, becoming the best at something. "Your Kung Fu is too weak for the overwhelming NPC stance!" wouldn't fly with me...
This is especially true of wizards.
And more often than not, if the player wants to commit to the background/training/ whatever, then they could eventually get the ability also.