  Violation of the $U(1)$ axial symmetry in QCD is stricter than the chiral
$SU(2)$ breaking, simply because of the presence of the quantum axial anomaly.
If the QCD gauge coupling is sent to zero, the strength of the $U(1)$ axial
breaking coincides with that of the chiral $SU(2)$ breaking, which we shall in
short call an axial-chiral coincidence. This coincidence is trivial since QCD
then becomes a non-interacting theory. Actually, there exists another limit in
the QCD parameter space, where an axial-chiral coincidence occurs even with
nonzero QCD gauge coupling, that can be dubbed a nontrivial coincidence: it is
the case with the massive light quarks $(m_l\neq 0)$ and the massless strange
quark ($m_s=0$), due to the flavor-singlet nature of the topological
susceptibility. This coincidence is robust and tied to the anomalous chiral
Ward-Takahashi identity, which is operative even at hot QCD. This implies that
the chiral $SU(2)$ symmetry is restored simultaneously with the $U(1)$ axial
symmetry at high temperatures. This simultaneous restoration is independent of
$m_l (\neq 0)$, hence is irrespective to the order of the chiral phase
transition. In this paper, we discuss how the real-life QCD can be evolved from
the nontrivial chiral-axial coincidence limit, by working on a
Nambu-Jona-Lasinio model with the $U(1)$ axial anomaly contribution properly
incorporated. It is shown that at high temperatures the large differences
between the restorations of the chiral $SU(2)$ symmetry and the $U(1)$ axial
symmetry for two light quarks and a sufficiently large current mass for the
strange quark is induced by a significant interference of the topological
susceptibility. Thus the deviation from the nontrivial coincidence, which is
monitored by the strange quark mass controlling the topological susceptibility,
provides a new way of understanding the chiral $SU(2)$ and $U(1)$ axial
breaking in QCD.
