Leg length difference
Leg length differences are probably present in everyone, but in many will be so slight that they will be of no clinical significance. As what stage a leg length difference become clinically significance and contributes to problems is not known and is the subject of debate and research. Some believe very small amounts of leg length difference are clinically significant, but others believe the difference has to be larger before it becomes an issue.
Resources:
Compensating for Leg length Differences
Foot Pronation and Structural Leg Length Differences