Developing time mainly affects highlights - Zones VIII and above.

Zone system metering determines the density of the negative. If you place Caucassian skin as Zone V (middle gray), and then make what Fred Picker called a 'perfect print' (a print which is exposed such that the blank film rebate prints as Zone 0) then Caucasian skin would appear to be Zone V. But you can print that same negative making the Zone V areas appear to be Zone VI by simply holding back exposure.

So I think what you are experiencing is crossover between film exposure and print exposure. You are underexposing your negatives (placing Zone VI subject areas on Zone V), and then compensating for that by printing so that the areas placed as Zone V as if there were Zone VI.