There are lots of theories, but no science to back it up. One theory is standing on the pedals, but I believe that body position (possible mechanical advantage) and muscles involved, plus weight of the bicycle, plus the training of the individual all make a difference.
If you take a brand new cyclistr on a decent DF bike, he/she can tap into at least one of those possible advantages I mentioned above and be faster on a hill climb on the DF. Put that person on a bent and it's such brand new territory that they have no choice to be slower.
Take that same guy and move him to a good performing bent, give them a lot of training miles and time to develop bent muscles, and he will be an equal climber.
I ride both bikes, and I'm fast on both. I compete in triathalons and such and finish in the top 10% on the bike easily. And what I know is that I am just as fast on the bent uphill as I am the df. However, a paceline or a well-trained triathalete will beat me up a long hill every time. On very short hills where I have speed on the bottom though, I will beat them all. It takes more energy (muscle AND aerobic wise) to do so on the bent. But I make up for it with less energy expenditure on the flats and descents.
Why all this is, I have no idea, but I've observed it time and time again.
Oh, during a pay ride last in one of the hilliest areas around here, I literally towed a DF rider up a couple hills. He put his hand on the back of my p-38 and had me help him.
