The DifficultyOffset scales from 0 to 1, but I don't think ASM has yet been updated to reflect the new 4.0 Difficulty scaling (400%) with lvl 120 Dinos. Anything above 1 is probably disregarded as invalid and ignored, defaulting to the default 1.0 Difficulty (100%)
Read more: http://arkservermanager.freeforums.net/thread/167/high-level-dinos-spawning#ixzz3q27ZdHaJ