It makes perfect sense, 100% of the entire game engine is simple calculus as it is impossible to code any differently. However, you haven't provided 20% of the required variables to even remotely calculate. Asking someone to explain the calculus behind the game dynamics in a single post isn't going to end well, these aren't abc's.
A 55 Mammoth that gets health damaged 20% does not turn it into a level 44 unit for offensive purposes, but 54 as power and health is exponentially greater with each level. A level 44 Mammoth has 120.5k health and does whereas a level 55 Mammoth has 343.7k health. 20% of 343.7 = 68.7k therefore Mammoth was dropped to 275k health. 275k health is level ~53.8. From here we begin discussing mammoth damage at the modified level vs. unit type.
Base damage for Mammoth vs. Infantry/Vehicle/Building/Artillery = 150/280/180/180 respectively. This is modified by level, unit multiplier, health, offensive poi's and BOTH the defending alliance's offensive & defensive poi's and each alliances overall ranking percentage score as well.
Level 55 Mammoth damage = * UM * H, where X is base unit damage, L is level, UM is unit modifier and H is health. UM for mammoths is 2.5. From here we would normally begin discussing the calculus behind how POIs work and overall affect of rank as compared to other alliances on server.
The short version is this; If the defender is higher ranked POI's than the attacker, a level 51 unit can defeat a level 55 attacker that was damaged 20% with ease.
A much shorter version is this: Level is King, period.