CR for a monster tend to be a little bit lower than it's of a NPC which have identical HD and abilities. A monster supposed to have average ability scores for it's race, and the items listed in the stats only. On the other hand, a NPC with levels supposed to be created by 25 pts buy (or standard array) and have equipments according to his level.
For example, a CR 10 Spell-Weaver do not have any magic items except for his disk and those randomly generated as treasures. On the other hands, a 10th-level NPC Sorcerer supposed to have 16,000 gp of gears and thus considered to be equally powerful as CR 10 spell weaver. Also, both of them supposed to have same Cha score (16).