In the minimal $SU(5)$ GUT model, the masses of the leptons and the corresponding down-type quarks must be equal. i.e. $m_e=m_d$, $m_\mu=m_s$, $m_\tau=m_b$.
Whereas this is approximately correct for some (e.g. muon mass = 105MeV and strange mass = 95MeV) there is still a difference espscially for the heaviest generation.
Is there some mechanism consistent with SU(5) symmetry that could account for the mass differences? e.g. mass running?
Only mechanism I could think of would be to postulate a complete copy of $SU(5)$ (mirror matter) with different masses. But then one would have to explain why we see only some particles from each sector.
Or is it likely the masses of quarks have been measured wrongly?