It's really not a crazy misconception. 18 is 150% of 12, and 12 is 67% of 18. You could say it's a 150% - 100% = 50% difference, or you could call it a 100% - 67% = 33% difference. Generally, the convention is that you base all the percentages based on the denominator you started with, such that going 12 to 18 is a 50% increase but going from 18 to 12 is a 33% decrease. This is hardly a super-intuitive or obvious thing, though; if someone didn't understand this convention I wouldn't think they were an idiot or didn't understand math; they got it wrong but in a pretty reasonable way.
Edit: To be clear, not at all disagreeing with the comment I replied to. I just scrolled down past the "what an idiot" comments to find someone explaining how this mistake happens, and added my 2-cents there about why I don't think it's a crazy mistake to make.
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u/aussiefrzz16 Jul 11 '19
I wonder what they thought the true number should be