Tar turns matt when its surface oxidises, loses lighter material, gathers fine dust, and becomes microscopically rougher.
A matt tar surface usually means the top layer has weathered. Fresh tar reflects light from a smoother, oilier film. As air, sunlight, rain, and dust affect that film, it becomes less smooth and less reflective.
Dust and pollen can matt the surface early. Tar that remains tacky for a while acts like a trap for tiny particles. Once those particles become part of the surface, the coating looks duller even if the tar beneath is still soft.
Oxidation and ultraviolet exposure also change the surface. The outer layer becomes harder, drier, and sometimes greyer. On old timber this can create a quiet, smoky finish that looks very different from fresh black tar.
Matt tar is often normal ageing. It becomes a problem only when the surface is also powdery, cracked, lifting, or no longer shedding water. Colour and sheen are clues, but performance matters more.