> Am I missing something?

If you’re sincerely asking…

Mathematically speaking, 18 / 3.6 isn’t “reducing” by 3.6X, it’s “dividing” by 3.6X. Reducing would be 18 - (18 * 3.6), which is obviously wrong. By your formula, “reducing by 50%” would be 18 / 0.5, also obviously wrong.

Yes, people do say things like “reduce by 3.6X” and are understood to mean what you said, but they also say “literally” when they mean “figuratively”. It doesn’t bother me but I can understand why math oriented people would be annoyed, and I personally would never say “reduced by 3.6X”, but instead “reduced by 72.2%”.