The cream dilution, which creates palomino, buckskin, brownskin and smoky black, is an incomplete dominant. These means it acts differently based on it's zygousity. I know, everyone knows this, chestnut with one copy is palomino, with two is cremello. However, a secondary effect of this incomplete dominance is that cream often has very little impact on black hair when it is heterozygous (only one copy present). This is why buckskins retain those black points so well - because the cream doesn't really cause it to change. There are exceptions to this, with some smoky blacks being really easy to pick for example, but the general rule of thumb is that cream x 1 doesn't change black. So of course, if you have a heavily sooty chestnut with one copy, it is perfectly normal to expect that the sooty will not be as diluted as the rest of the horse is.
This guy is a palomino, genetically tested. The dark mane is caused by sooty.