Species similarity can imprint abundance fluctuations in two opposing ways: similar species may respond similarly to environmental fluctuations, generating positive correlations, while stronger overlap in resource use enhances competition and generates negative correlations. In the absence of additional mechanistic information, the natural benchmark is therefore to match environmental-response similarity to competitive similarity. At this matched point the two effects cancel exactly and equal-time interspecific correlations vanish, so nonzero correlations primarily measure departures from matching rather than niche overlap itself. Making resource dynamics explicit reveals two natural ways to break this cancellation. A temporal mismatch arises when environmental response is immediate but competitive feedback is delayed, allowing correlations to track niche overlap when the relevant timescale is known. A structural mismatch arises when fluctuations act through resources: forcing then depends on yield, whereas competition depends on both yield and depletion, so correlations reflect yield-depletion mismatch. Equal-time correlations are therefore informative only relative to the mechanism that drives the system away from the matched benchmark.