Here's how 'Journal of Marketing' explains it: Let's say we're dealing with 10 bananas for $5. In the first option, you'd get 50% MORE for the regular price, or 15 bananas for $5. That's 33 cents per banana.

In the second option, you buy the regular quantity at 50% OFF, so you'd get 10 bananas for $2.50 or 25 cents per banana.