Sherpa, I've done that, and it is indeed strange. I was free diving to 35 feet, checking out a C-47 that lies about 200 yards out in front of La Cieba in Cozumel for 30 seconds or more, and then slowly ascending back to the surface. My time under water was in excess of 2 minutes when normally I do well to hold my breath for 1 minute.
The thing i figured out was that the more you do it in succession, the longer you can hold your breath. The other side of that is that the air in your lungs compresses, so on the bottom, you have less of it. You can stay down until you are completely out of air, and as you ascend it's like getting a new breath as the air expands in your lungs. I couldn't explain it either, but I could do it. There are quite a few good fossil spots in less than 20 feet off Vero beach, I would imagine with a little practice, I could spend a full minute searching on the bottom. It's strange, but it works...I extended my time by around 10 seconds each time I went down.