This result could change physics forever

New measurement of muon G-2 is a remarkable achievement.  However "new physics which we don't know" is not the only possibility. Another one is that there is an *old* physics which we don't know: low-energy QCD (and in fact any non-perturbative quantum field theory). QCD contributions are negligible at the energy scale of the electron mass, that's why the theoretical prediction of electron G-2 is in such a nice agreement with experiments. On the other hand muon is ~ 200x more massive, and QCD corrections (loosely speaking bumps from virtual protons, pions, and other strongly interacting particles) are more important. And we can't compute those from the first principles. So disagreement between the theory and experiment is not that surprising.