I definitely have fast bikes and slow bikes, but I don't understand why, either. My Lemond is my fastest. So here's a theory. When it has the 700c wheels, they are light, and the tires are narrow and light. I pull away from a stop, and the lightweight wheels make the bike seem light. Then when I get up to cruising speed, I happen to be in an aero position because the bars are lower than on my other bikes. At that point, it's the aero advantage that makes it fast. I get seriously better times on it than on my other bikes. But another factor is probably weighing in. Because it's fast, it's comfortable to ride at a higher level of effort. So it's not that it gives me more speed for a given effort. Rather, it encourages me to put in more effort. And the increased effort makes me go faster.

Does this make sense? It's all conjecture.