It's the same reason that the nominal thrust level on the Space Shuttle Main Engines is 104.5%[1]. No, not 100% (it was originally), 104.5%. Why? Because you d

It's the same reason that the nominal thrust level on the Space Shuttle Main Engines is 104.5%[1]. No, not 100% (it was originally), 104.5%. Why? Because you don't go back and change all your rules and documentation following subsequent developments in the field, that causes unnecessary confusion and errors down the road. [1]: https://en.wikipedia.org/wiki/RS-25#Engine_throttle/output https://en.wikipedia.org/wiki/RS-25#Engine_throttle/output