Lichtl says that people have tried to use wavelet compression before, and these particular simulations are based on work done by Jonathan Regele, a professor at the department of aerospace engineering at Iowa State University.

“The difference is that without GPU acceleration, and without the architecture and the techniques that we just described, it takes months on thousands of cores to run even the simplest of simulations. It is a very interesting approach but it doesn’t have industrial application without the hardware and the correct algorithms behind it. What the GPUs are doing here is enabling tremendous acceleration.

via Rockets Shake And Rattle, So SpaceX Rolls Homegrown CFD.

To be more precise, if you get the temperature wrong in the simulation by a little, you get the kinetic energy of the gas wrong by a lot because there is an exponential relationship there.  If you get the pressure or viscosity of the fluid wrong by a little bit, you will see different effects in the nozzle than will happen in the real motor.