We don't know when she became exceptional, or if monster leveling works the same as character leveling.
Also you do realize that the opening panels of the entire series show her on basically equal footing to the Demon Lord, right? Who is the only other level 99 monster shown so far. Taking a lowly monster like a goblin and just making it the same level as the most powerful boss wouldn't actually make them anywhere near each other in strength. Even at the same level, a weak monster class would be nowhere close to the power of a major boss class monster.
So it was kind of clear she was exceptional in some way beyond level from the very first panel.
Not to mention her backstory pretty much outright mentioning how absurdly exceptional her luck was.
The premise you're claiming it kills never actually existed in the way you seem to think it did.