If it’s hitting lead and then going on to the tiles it’s the lead that’s cleaning them,look at directly below lead flashing or on a dormer window they all have a stretch of clean tiles from water that runs straight off the lead.
Something to do with a chemical reaction with lead and water I said to a roofer once if you ran a stretch of lead across the top of the ridge tiles on a roof they’d have clean tiles for years,he looked at me and said yeah they would lol.