The game guide has some explanation on the difficulty levels:
https://us.diablo3.com/en/game/guide/gameplay/game-difficulty
After Torment 1, the health and damage of monsters scaled linearly.
Other reference: http://d3resource.com/difficulties/index.php?1
Yeah, I get the differences in Game Difficulties. But, I thought CoeXane was asking how that related to Greater Rift Levels.
In other words, I thought they were asking why GR10 through GR12 equals Torment Level 1.
Maybe I misunderstood what they were asking. 
Edit: Although, D3 Resource does show both, Game Difficulty and Greater Rift Level. (I almost missed that. 
)
For every GR rank increase, mobs get a 17% Hit Point increase, and that increase is cumulative, i.e. a ten GR increase means mobs get a (1.17)^10 HP increase, which is 4.8 times. So, in order to be able to successfully clear a GR 10 levels higher, you need to be able to do 4.8 times as much damage.
It sounds like your mathematics is quite calm, but could possibly explain how d3 scaling works on a baby level. Maybe how i can upgrade my items to the precise degree that i can beat the scaling itself, beating the games difficulty entirely.