It is interesting, but note that I made sure to specify "anecdotally". To get real measurements, you would need a calibrated volt/amp meter with a data logging function.
The extent of my measurement is a plug-in voltage gauge and/or plugging in my GAP Tool.
I know empirically that my factory alternator liked to hang out around 12.9 - 13.4 volts. If you disconnected/reconnected the BMS or reset the battery it would temporarily run higher, but would always settle back to 12.9 - 13.4 over the course of a few days. I know empirically that I have had the new alternator in for a few months and I have yet to see a voltage indication lower than 13.9.
What conclusions can we draw? None really, because we don't have good data. We know in general that as voltage decreases amperage increases for given amount of work or watts or in this case whatever the truck needs. That would lead me to believe, in general terms that the factory alternator was working at a higher percentage of ampacity of the alternator (voltage lower at 12.9 and amperage based on the needs of the truck). As the load required by the truck is the constant in this situation, it would stand to reason that the new alternator is working at a lower percentage of ampacity as the voltage is consistently higher and the needs of the truck haven't changed (Voltage higher at 13.9, needs of truck stayed constant, so amperage has to be lower).
This all before you get into the added complication of what the damned BMS system is telling the charging system to do.
At the end of day, for whatever amperage, I prefer my AGM batteries to be seeing 13.9 volts charging input rather than 12.9 so I'm happy with the change.