Comment by tzs

I think they mean that what happens when a neutron decays is well understood. One of the neutron's down quarks change to an up quark, facilitated by a virtual W boson with negative charge. The W boson is very unstable and immediately decays into an electron and an electron anti-neutrino, both of which are ejected leaving behind the former neutrino which is now a proton because of that quark change.

When that happens is less understood, hence the discrepancies you mentioned.