He got it before he was a great practitioner of magic. Also, Pug is humble, partially as a result of seeing what the abuse of power did on Kelewan, where the magicians of the greater path did a lot of damage, simply because they had the power to do so. He abandoned the name Milamber, and went back to Pug, because that's who he was. I think Feist is making a point with this. Pug is more than a great magician. He is also a great man, because he does not let himself get full of himself.