They all seem to brown out to some degree, just some do it more or less. From what I've seen super cinnys tend to be more brown compared to super black pastels. But that doesn't always hold true as a blanket statement.
I've heard quite the range of ratios of problem animals, anywhere from 1 in 50 animals to 1 in 3 animals. Assuming people aren't lying I would assume there is either external factors at play or polygenetic traits at play. Neither could be calculated.