Am I missing something here? What's the difference between 'high luminance level' and 'high luminance', the article states:

"When the luminance level was high people were more likely to ... go for the certain $5."
and also
"On high luminance days, they were more likely to go for an unknown chance of getting $20 over the certain $5 payout."

@ kuncoro - the first case refers to KNOWN risks (e.g., a 50% chance of $20):

When the luminance level was high people were more likely to avoid known risks. When offered a choice between a certain $5 payout and a 50 percent chance of $20, they were more likely to go for the certain $5.

The second case refers to unknown risks:

Surprisingly, they had greater tolerance for unknown risks. On high luminance days, they were more likely to go for an unknown chance of getting $20 over the certain $5 payout.

Apparently on dark days people don't like an unknown chance, and on bright days they prefer an unknown chance to either certainty or a known chance.
I agree that it is weird, but the article itself agrees that it is surprising.

Let's see if this result is confirmed by future work...