Kingreaper
Adventurer
Many valid points already made.
Very often D20 isn't about degree of result but is a hit target number / missed target number issue. Aside from the simple math being done wrong, it was the wrong math to be doing.
Umm, no.
The logical conclusion isn't:
"Their simple math was wrong, and they were doing the wrong math anyway, which just happened to result in them getting the right answer by coincidence"
It's:
"They were doing the right math, not the simple math at all, and got the right answer"