I have observed the same thing. Drink skim milk, diet soda and then ear ice creams, burgers and fries. It's to lie to themselves, I think.
"I'mma have a soda. Coke and Coke Zero have equal flavor value for me. Might as well get the Coke Zero."
"I'mma have a side dish. Kale and Fries do not have equal flavor value for me. I'll get the fries."