That is because of the nature of the damage mechanics calculated by the game.
For our argument let us take a hypothetical Kage with attack stat 750, and a hypothetical Rana with defense stat 700.
For >50% Health, Kage Attack Stat = 750x4.5 = 3375, Damage = 100x(3375/700)^1.35 = 836
For <50% Health, Kage Attack Stat = 750x2.25 = 1687.5, Damage = 100x(1687.5/700)^1.35 = 328
We can observe that 328/836 ~ 39%, which similar to what we see in your screenshots: (321/731) ~ 44%, and not 50% that generally think going by just reading the numbers in the specials description.
The difference comes because there is random parameter inside the damage formula which changes every time damage is calculated. Hope this helps