[SixOfSpades]

There are multiple "incarnations" of each NPC. Each "incarnartion" has a different EXP level, and those with more EXP have weapon proficiencies and Thieving skills to match their level, and their Inventories and Mage Books change a bit as well. (Example: Pick up Kagain as soon as you can, and he'll be wearing Chain Mail. If you wait until much later, he'll have Splint or even Plate. And depending on when you recruit her, Dynaheir may or may not know Fireball.)

Which of these "incarnations" you actually get depends on your level: The game checks your EXP, and assigns whichever incarnation most closely matches that EXP. Most NPCs have no incarnations of a level higher than 6.