Assuming that you have indeed confirmed that it's the speaker and not something else, that is, you've swapped them left for right and the sound follows the speaker, then there are only a couple of possibilities:
1. Damaged speaker - a failure in the crossover network could easily cause what you describe, but occasionaly a driver failure will cause it as well.
2. Manufacturing tolerances - just because they're consecutive serial numbers or a supposedly "matched pair" doesn't mean they were perfect to begin with. Speakers are made by people, and people make mistakes.
1. Damaged speaker - a failure in the crossover network could easily cause what you describe, but occasionaly a driver failure will cause it as well.
2. Manufacturing tolerances - just because they're consecutive serial numbers or a supposedly "matched pair" doesn't mean they were perfect to begin with. Speakers are made by people, and people make mistakes.