TC, Shugart, and Davies have written about them.
I was asking why it's easier with the barbell.
The Pavelian Pucker Theory seems to have some merit but I should be able to get the same benefit from applying the Pavelian Pucker to the dumbbell version.
Better aerodynamics doesn't explain it because I workout in a vacuum-sealed room while wearing an oxygen mask.
Comparing the curl to a bench press is a poor example. A barbell curl is less stable than it's dumbbell counterpart while a barbell bench press is more stable than a dumbbell press.