  There has been much speculation as to the origin of the \Delta I = 1/2 rule
(Re A_0/Re A_2 \simeq 22.5). We find that the two dominant contributions to the
\Delta I=3/2, K \to \pi \pi{} correlation functions have opposite signs leading
to a significant cancellation. This partial cancellation occurs in our
computation of Re A_2 with physical quark masses and kinematics (where we
reproduce the experimental value of A_2) and also for heavier pions at
threshold. For Re A_0, although we do not have results at physical kinematics,
we do have results for pions at zero-momentum with m_\pi{} \simeq 420 MeV (Re
A_0/Re A_2=9.1(2.1)) and m_\pi{} \simeq 330 MeV (Re A_0/Re A_2=12.0(1.7)). The
contributions which partially cancel in Re A_2 are also the largest ones in Re
A_0, but now they have the same sign and so enhance this amplitude. The
emerging explanation of the \Delta I=1/2 rule is a combination of the
perturbative running to scales of O(2 GeV), a relative suppression of Re A_2
through the cancellation of the two dominant contributions and the
corresponding enhancement of Re A_0. QCD and EWP penguin operators make only
very small contributions at such scales.
