Why doesn't my s-log3 footage reach 100% value?
A log curve is a mathematical transfer function.  SONY have defined a LOG curve (as many vendors do) that imagines future/alternative sensors having larger electron well capacity (higher clipping) and lower noise (lower clipping) than your current silicon design supports. In other words, you are seeing where your sensor's designs fits into an idealized LOG space.  You are not seeing a LOG curve that is limited by only what your particular sensor can achieve at the present moment.