Sparsha names touch as a subtle quality prior to any tactile event, the tanmatra vayu will eventually carry once enough grossening has occurred for air to condense at all. Where shabda alone is sufficient to constitute akasha, sparsha never appears without shabda already present, since Samkhya's account of grossening is cumulative rather than a set of five independent substances: each new element inherits every tanmatra established before it and adds exactly one of its own.
This is why vayu is traditionally credited with carrying both sound and touch, rather than touch alone, an attribution that follows directly from sparsha's position as the second tanmatra rather than from any observation about how air actually behaves. The tattva sequence is doing genuine explanatory work here: it is not merely describing air as touchable, which would be a fairly trivial observation, but deriving that touchability, together with air's audibility, from a fixed prior ordering of subtle qualities.
Sparsha's corresponding organ, tvak, registers this one tanmatra and no other, the same discipline every Jnanendriya observes toward its matching tanmatra. The correspondence between sparsha, vayu and tvak is therefore not a loose thematic grouping but a specific claim about what is ontologically prior to what: the subtle quality first, the gross element that carries it second, and the organ built to detect that quality a parallel but independent third production, all three ultimately traced back to a single tattva, ahamkara, by two different guna-differentiated routes.