Might I suggest one other way of looking at this data visually. Your method, I believe, magnifies the differences. How about graphing the points vs time. So, each person would have two points, one at 5 seconds and one at 3600 seconds. Then draw a line between the two. I think you would find that the slopes of the various lines, while having some differences, would be quite similar.

The question that is unanswered, it seems, is how to fully explain the differences?

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Frank,
An original Ironman and the Inventor of PowerCranks