Rate of usage and level of the enemy you're facing affects combat skills and offensive magic. As for magic like enhancing and healing, generally comes down to rate of usage and possibly some other factors I really don't know... mostly luck I guess.

Combat skill example
If you're always actively engaged in combat, of course whatever weapon your using will get capped because you're swing more anything else. If you're solo, evasion is always going up, if you're in a party and not holding hate as much then naturally your evasion skill goes up slow. But generally speaking your hitting and evading more hits than guarding and parrying. These start to stagnate as you level and progress to stronger enemies. In order to skill up effectively, you have to skill up on monsters at around 1/3 of your natural level. You won't skill much, if at all, if you're jumping fighting a level 80 monster with only 120 to say parrying.

Offensive magic works the same way too, only you can imagine that you'll never cast more spells than any melee swinging a weapon.