Why don't Burne and other NPC companion Wizards know all the spells they should?

Consider this: Burne is a level 8 Wizard (initially) who slew a juvenile green dragon with a bolt of magic. Magic missile checks for that. I get that.

Why, though, does he not know anywhere near the repertoire of spells sold as scrolls from his own vendor? Also, why does he not already have in his spellbook/repertoire the spell scrolls he starts the game with? He doesn't seem lazy and his INT 17 doesn't make him stupid.

Next, why does Burne only know 2 level 4 spells initially when, at level 8, he should have "really" picked 4 (instead of 3) level 4 spells? Why is this also true of level 2 spells? (I recommend stoneskin and fox's cunning as extra spells known.) Burne is even lacking in level 1 spells, lacking either 1 or 2 depending whether he put both level up points (from level 4 and 8) into INT. (He probably put both, meaning shield and grease are good candidates.) I can understand not starting the game with 300,000G worth of stuff for balance reasons, but not having all his class features just feels bad.

Pishella - who takes after he master? - is missing 2 level 1 Wizard spells known.

Sargen and Spugnoir - other Wizard companions - have this problem of missing spells. Spugnoir should know 8 level 1 spells (3 + INT mod at level 1 + 2 at level 2) and Sargen should know 8 level 1 spells (for the same reasons as Spugnoir), plus 2 more level 2 spells! (I recommend mirror image and glitterdust for immediate usefulness.)

So, wuzzup?