The ratio T₂/T₂^* of the Hahn-echo and Ramsey times of nitrogen-vacancy (NV⁻) ensembles is independent of the nitrogen concentration [N] and is ≈ 16 in diamond of natural isotopic abundance (¹⁴N), yet a recent ¹⁵N-doped ensemble magnetometer gives only 8.05 ± 0.18 in the single-quantum convention. We ask whether the nitrogen nuclear isotope alone can cause such a reduction. Using a semiclassical spin-bath model with the Jahn-Teller-resolved P1 hyperfine tensor, we show that the fraction of P1 pairs that are hyperfine-degenerate, and hence free to flip-flop, is 1/4 for ¹⁴N (I = 1) but 5/16 for ¹⁵N (I = 1/2). Simulations over [N] = 0.1–100 ppm confirm that this shortens T₂ while leaving T₂^* exactly unchanged: T₂(¹⁴N)/T₂(¹⁵N) = 1.118 ± 0.011, independent of [N] and 11σ above unity. The measured contrast, 2.07 ± 0.25, is 3.8σ larger, identifying resonant-channel counting as a real but partial contribution and setting a quantitative benchmark for many-body calculations.