When relative humidity is 100% the air can't absorb any more moisture and it becomes increasingly difficult to stay dry. However, the outer surface of a garment and the boundary layer next to it, can be slightly warmer than the atmospheric temperature. This boundary layer can therefore absorb moisture from the garment and be whisked away by the wind or even drafts caused by your movement. Unlike an aircraft wing, for example, fabric flapping may help dislodge this layer. This process requires body heat to drive it and once you become chilled less heat is available.
Wind can also reduce the pressure over parts of your clothing, improving breathability. Please see this post.
In the rain, when the outer fabric is saturated, the inside is very likely to reach the dew point, causing condensation. This post explains how FurTech can remove condensation from your clothing, unlike membrane garments.
When relative humidity is less than 100%, breathability can be driven by greater humidity inside the garment and moisture diffuses through the fabrics.