The $79,000 figure is an average. I doubt a single participant said they wanted exactly that much money to be happy. The part of the study I could read even mentioned that they controlled for demographics (age, where you live, etc). $79,000 per year won't go far in Los Angeles, for instance, but would go far in many parts of the US.
The quoted report is behind a paywall, and I couldn't figure out if they meant individual income or household income. $79,000 of household income, while above the median, is not ridiculously hard to get.
The other one I've heard is everyone thinks if they just made 10% more money, they'd be fine.
Income: $10,000 per year. Expenses: $9,999.99 per year. Result: Happiness.
Income: $10,000 per year. Expenses: $10,000.01 per year. Result: Misery.
I think it's this. Human beings think short term. The income figure has less impact on a human mind than on how much money is left at the end of the month. If their bank account and investment account is increasing in size, most people would be happy, as it implies security (you're not likely to go into the hole). Of course, that can be weak thinking (your emergency fund might be too small, so any little thing could drop you into the hole).
That's not a very Mustachian attitude, however. We think long term.