I suspect it's affinity exploitation - wherein minority salespeople take advantage of the ignorance of the minority clients they serve. This kind of thing shouldn't be difficult to flag given the fact that everything is computerized these days. There'd be less of this type of discrepancy without affirmative action in hiring, since incomes would track ability, rather than today's reality, in which legally-mandated racial preferences promote favored minorities above their innate abilities at every level of employment. Nonetheless it should be possible to companies to filter for this kind of affinity exploitation. The downside is that protected minorities will also have higher default rates at every level of income, since the ability to handle money is correlated with innate intelligence, and most protected minorities make more money at a given level of IQ than their white or Asian counterparts, thanks to Federally-mandated racial preferences.
In Wells Fargo’s case they just fired their whole independent channel and decided to keep everything in house where they could better monitor pricing.