First of all, that’s five standard deviations off of the expected value. On a practical level, that means that of all the variability in every single measurement made of the size of the proton in the history of time, this new measurement is so damned far off that the guys doing the experiment didn’t even notice it the first two times (2003 and 2007) they measured it.
Anyhow, something is amiss. Most likely, this is not the sign that everything we know is wrong, or that the powerful equations of quantum mechanics are all wrong (as evidenced by the 20th century). Rather, this is probably a sign of some new interaction between muons and protons we haven’t seen before. That means new particles, and new physics. Wahoo!
Note: I can’t link you directly to the report from Nature, because you can’t access the information because we live in a stupid, fucked up world.