= Solution
The important change is \b[preorganization of the chloride-binding <amide> groups]. In the hydroxylated host, intramolecular <hydrogen bonds> from the phenolic OH groups to the <amide> <carbonyl> <oxygens> restrict rotation and favor a geometry in which the <amide> NH donors converge on the anion-binding cavity. <chloride> can therefore accept two favorable <hydrogen bonds> with a smaller conformational penalty than in the unsubstituted host. The hydroxyl groups need not be counted as two additional freely available <chloride> donors: their role includes organizing the existing donors.
The <association constant> increases by $5230/195=26.8$. At $298\,\mathrm K$, the corresponding difference in standard <Gibbs free energy> is
$$
\Delta\Delta G^\circ=-RT\ln(5230/195)\simeq\boxed{-8.15\,\mathrm{kJ\,mol^{-1}}}.
$$
This is a modest molecular free-energy advantage amplified into a substantial change in the <equilibrium constant>. <solvent> competition and changes in <solvation> also contribute to measured binding; the structures chiefly suggest the preorganization explanation.
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