I've wondered about this for years.

My best guess is that your body just has different states depending on how well you slept recently, what you've been eating, where you are in your digestive cycle, and what your mental state is.

If your body thinks you haven't eaten enough lately, it's going to try to tell you, in the only way it knows, to slow down and conserve energy. It will make you tired.

Sometimes I've found that when riding with someone slower, I'll get more tired, and when trying to go faster, as during an organized ride, I'll have more energy. It's a puzzle.