Ok, so Curse gives an attack boost of 50%. Basically, look at this.
252 Atk Landorus-Therian Stone Edge vs. 0 HP / 0 Def Dragonite: 248-292 (76.7 - 90.4%) -- guaranteed 2HKO
This is before an attack boost. (Yes I understand that landorus-t doesn't get curse but it can be simulated by simply boosting the attack stat by one stage lmao). Now, look at this:
+1 252 Atk Landorus-Therian Stone Edge vs. 0 HP / 0 Def Dragonite: 370-436 (114.5 - 134.9%) -- guaranteed OHKO
Focus on the percentages. 76.7 is around 115, which 114.5 is close to (other modifiers are included). The same goes for 90.4%. This is around 137, which is decently close to 134.9. Again, not perfect, but close enough.
This is the calculator I used