Each individual athlete has their own set of constraints. For example, some pitchers might have poor hip flexibility, or thoracic spine mobility, or perhaps tight groins or hamstrings. Additionally, each athlete will have his own set of strengths. Perhaps he has no flexibility issues, or he is tremendously strong. The individual pluses and minus that a pitcher has is going to affect his movement patterns and the way he innately generates momentum, force and power. As we should all know by now, no two individuals perform the same movement the exact same way. Most people find out how they are able to accomplish a movement or exercise and it is slightly different than others performing the same movement.
With that being said, we cannot expect every pitcher to look identical when trying to generate velocity. Some pitchers are able to generate velocity simply because of their size and brute strength, while others may generate the same velocity through momentum and tremendous flexibility. One only needs to think of Roger Clemens (above) and Tim Lincecum for that illustration. Both can throw 95+, yet generated that velocity in different ways.