Your damage calculations are incorrect. You assumed that Deoxys-S has maximum defenses and HP.
The EVs that you have give it defensive stats of 304 HP, 216 Defense, and 216 Special Defense.
For example,
actually is
307 attack vs 216 defense, 120 power(* 1.3 * 1.5), 304 max HP: 78.95% - 92.76%