Why do experts disagree on the impact of soiling and soiling rates?

That is a really good question. We often review “scholarly” articles and studies which all claim to have factual evidence-based data on soiling rates. From this data, they often postulate that washing does not need to be done very often and some not at all.
If these studies are so factual, why do they disagree on soiling rates? In all fairness, we do note that most studies do address what they refer to as the exception, citing environmental factors we know can make a tremendous difference on soiling rates. And, soiling rates are less where the panels are mounted at a greater angle and in areas that are less dusty and have more frequent rain.
Our experience is that soiling rates tend to be very different from location to location. Locations that are nearby can often have different soiling rates. Some of those reasons are seasonal, proximity to dusty environment (desert, agricultural, pollen, freeways, airports, wind patterns) and they can also be affected by the mounting angle of the panels. The flatter the angle, the worse the soiling rates.
Often these papers cite rain being sufficient to clean “most” of the soiling; interesting. Now picture your car sitting in the weather for 25 years letting it be washed only by rain. How well do you think that would work? As I have noted before, a solar panel is simply a 3 mm. thick glass window with the “magic” occurring below. If the sunlight is obscured then less power is produced.
You need to keep your panels clean to maximize your investment.