Yo, I think you miscalculated the amount that Venomoth can do to Delphox at +1. You didn't give Venomoth any SpA EVs, which it usually runs. See below:
+1 252+ SpA Life Orb Venomoth Bug Buzz vs. 0 HP / 0 SpD Delphox: 246-290 (84.5 - 99.6%) -- guaranteed OHKO after Stealth Rock
+1 252+ SpA Life Orb Venomoth Bug Buzz vs. 0 HP / 0 SpD Delphox: 246-290 (84.5 - 99.6%) -- guaranteed OHKO after Stealth Rock