From the dungeon master's guide page 30: "For an NPC with an NPC class, determine
her Challenge Rating as if she had a PC class with one less level.". The orc warrior 1 is written as being CR 1/2 (and not 0), and the orc warrior 10 is CR 9. This clearly shows that reducing CR below 1 creates progressively smaller fractional CR and subsequently increasing CR makes the fraction go closer to 1. This is also clarified in Pathfinder as 1/8->1/6->1/4->1/3->1/2->1.