Well, the "dirt" or discoloration on a roof is not dirt at all, it is a plant, actually a Cynobacteria, that is growing there.
It takes time for this plant to regrow, and that is how warranties are given.
The roof cleaner is counting on it taking longer for this plant to regrow then the warranty period.
But, the growth of this plant is effected by a lot of factors beyond the roof cleaners control, like moisture and rain.
