= Solution
<Quark mass> differences break <flavor symmetry>[flavour symmetry]. <Isospin> exchanges $u,d$, so its strong breaking is controlled by the small difference $m_d-m_u$. <U-spin> exchanges $d,s$, so its strong breaking involves the much larger $m_s-m_d$. Relative to the characteristic strong-interaction scale, the latter is much larger; this explains why <U-spin> is a substantially poorer symmetry.
Flavour-dependent interactions are a second source. <Electromagnetism> distinguishes $u$ from $d$ because $Q_u=2/3$ and $Q_d=-1/3$, and therefore breaks <isospin>. However, $d,s$ both have charge $-1/3$, so their <electromagnetic interaction>[electromagnetic couplings] alone do \b[not] break <U-spin> in the equal-mass limit. If two breaking mechanisms for each symmetry are required, the second common mechanism is the <weak interaction>: the <CKM matrix> and weak <quark flavor>[flavour] transitions distinguish the flavours and do not conserve either strong-interaction <flavor symmetry>[flavour symmetry]. The dominant difference in the quality of the two approximate strong symmetries is still the <mass> splitting.
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