Challenge Ratings - Monster Codex - Giants

[All Seeing Archer]

So, I've been parsing through the Monster Codex I just received for Christmas, and found a couple odd CRs while going through the Giants section, specifically with anything that has a casting class. It appears that all Casting Classes are counted as +1/2 CR instead of the full +1 CR. I wanted to use different kinds of giants in an upcoming campaign, and it seemed perplexing that these CRs were so far off from the expected. Could someone elaborate on why the numbers are as follows?

Base Fire Giant CR: 10
Fire Giant Magmablade (14 levels of Magus): 17
Expected CR: 24
Fire Giant Doombringer (16 levels of Oracle): 18
Expected CR: 26
Fire Giant Queen (5 levels of Fighter/8 levels of Sorcerer): 19
Expected CR: 23

Base Frost Giant CR: 9
Frost Giant Ice Mage (6 levels of Sorcerer): 12
Expected CR: 15
Frost Giant Battle Priest (8 levels of Cleric): 13
Expected CR: 17
Frost Giant Houndmaster (10 levels of Druid): 14
Expected CR 19

[Falcar]

The general reason is because a frost/fire giant would be better suited as a melee type than a caster type. As a wizard the Giants high strength and melee suites powers are not helpful and (for the most part) being wasted.

[Chemlak]

See here. And here.

Basically, adding levels in an "out of role" class bumps CR less than adding levels of an "in role" class. Since all Giants are basically combat role creatures, adding spell role classes is cheap in terms of CR.

[A_psychic_rat]

also i feel it should be pointed out for monsters whos CR is base off of anything other than class levels dont gain 1 CR per class level automattically.

[All Seeing Archer]

Thanks all, that's a huge help!