# How do we know that weak isospin is an exact symmetry while strong isospin is not?

It is said in textbooks that if the $SU(2)_f$ or $SU(3)_f$ flavor symmetry were exact for sstrong isospin, then all members of the multiplets would be exactly equally massive. By looking at quark masses then one can conclude that the symmetry is not exact. But how does the argument work with the weak force ? It is said to be an exact symmetry, yet electron and neutrino have different masses even though they are in the same doublet. What is the difference ?