What is the significance of 104% for throttle power and rotor speed? I noticed this initially when learning to fly Robinson Helicopters but fail to remember the reason my instructor gave me; this again came up when watching a video about the ISS.


In helicopters (Robinson R22/44), the main rotor typically operates at 104% RPM.
On the space shuttle, after max q, they throttle back up to 104%.


What is the significance of 104%? If this is the maximum, why is this not called 100%, and everything scaled back to fit? This seems the most fit place to ask this, as I have only seen this in aviation / aerospace.
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.