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The problem is water trapped by capillary action.
Between slates…
Capillary action is the force that drives water up between very close surfaces. The closer the surfaces, the stronger the force (fig. 1).
While exposed slate surfaces dry in minutes, inspection reveals that covered areas, where capillary forces trap water, can remain wet for hours (figs. 2, 3 & 4). Weeping reveals the presence of trapped water (figs. 3 & 4).
Long after a rain, 60% of slate surfaces can remain wet as they rest, in effect, in their own puddles.