What is the significance of 104% for throttle power and rotor speed?
This happens when the maximum output was changed after design.

100% is simply a reference to a certain value. In the space shuttle's case, the engine output was increased after the initial design. Instead of updating the engine value everywhere, they decided to simply keep the initial value constant.

The same goes for N1 in turbo engines. 100% N1 is simply referencing a certain datum defined by the manufacturer. Going over 100% is not necessarily unsafe.