I can answer that and I though it has been answered. It's relative. A player turns 100% when he hits the 6* for your level. Each individual skill and player quality as a whole is internally still tracked in points and expressed in %, which in some senses is more logical, and in others less so. For any given level, 100% has a specific corresponding numeric value, for example, for level 8, 100% is 60 points. If any skill progresses past 60, it will be expressed as >100%. No superpowers, just normal progress toward the next star.
Thanks for the explanation but still there is a mystery about %.
For example the numbers of my player in post #8, the column with the defense % numbers