I don't really know if its only Staraptor or others but I'll try.
And here is a picture of the damage formula:
https://m.bulbapedia.bulbagarden.net/wiki/File:DamageCalc.png
Fight between Heracross-Mega and Choice Scarf Poliwrath:
252+ Atk Heracross-Mega Close Combat vs. 0 HP / 4 Def Poliwrath: 292-345 (90.9 - 107.4%) -- guaranteed OHKO
252 Atk Poliwrath Liquidation vs. 0 HP / 0 Def Heracross-Mega: 100-118 (33.2 - 39.2%) -- 99.8% chance to 3HKO
