It doesn't matter what the secret math is behind the scenes. Obviously there is a penalty of some sort that makes 115% less than 100%. The point is that the penalty is not reflected in the calculation Evony shows on screen.

If 115% of gems at star level 12 is actually 80% chance of success, then the screen should say 80% chance of success. You should be able to keep adding gems to get to 100% or more. People spend money on gems and deserve some honesty about what they are PAYING for.