As long as you aren't getting headaches or feeling nauseated after your dives, just consider yourself lucky! There ARE data showing that some people become rather CO2 tolerant when on scuba. Since CO2 is the major drive for respiration, CO2 tolerance results in lower RMVs -- but this can be a significant problem on deep dives, as CO2 and nitrogen are additive in their narcotic effects. As I recall, a death on the Doria recounted in one of my books occurred in a diver who was known for skip-breathing to extend his bottom time. He was found with empty tanks and no sign of struggle at all, and it was hypothesized that he was simply incapacitated by narcosis.
But some of us just fall into a very efficent breathing pattern fairly naturally. Although I am small, which is an advantage, I have always had a low gas consumption rate. It was the only thing in scuba that I did right from the beginning!
(Irritates the stink out of your buddies, doesn't it?)