I think it's skewed due to average stage length - i.e. longer flights have lower costs per mile and lower fares per mile. The difference between UA's average stage length vs. regional carrier is larger for UA than other legacies because of UA's Pacific presence.

Also rev per ASM for the regional carriers is normally based on contracted pricing with the legacy carrier (i.e. UA pays Mesa a fixed fee to operate the flights, and keeps all the passenger fare revenue). So this data likely understates what people actually pay to fly on regionals (in profitable regional markets that is).