I get that too. Some days I just feel better than others. I think some of it comes from riding when I'm not fully recovered from a prior ride/workout. My legs are just tired. I just ease back and do a recovery ride when I feel that way.

Putting that issue aside, there are some stretches of road that just slow me down. I assume it's the road surface. I thought it was just me at first but then I started noticing a pattern on my usual routes. There's one stretch of road in particular that makes me feel like I'm riding in quicksand everytime I'm on it. Beyond that, wind, cold and terrain can make your legs feel tired and slow you down.