On a Mopar style dual field alternator, what happens when you swap the field wires from the stock setup?

The reason that I ask is because when I took my Mopar square back alternator (left below) into NAPA to have it tested, they tested it as the chart shown and it failed due to lack of voltage. Then I handed them my Powermaster (right below) unit and they hooked it up the same way as the square back and the machine told them to swap the field wires. Once they swapped the wires, then it tested PERFECT.
I just received an email from Powermaster discussing my current ammeter/voltage guage fluctuating at idle and they are suggesting that I swap the field wires.
Why would they suggest this and why did the PM alternator test fine at NAPA after they swapped the field wires? I have been running this alternator for a year now (wired like my stock unit) with no issues until a few weeks ago. I am just trying to understand the suggestion.

EDIT:
This is the response from Powermaster as to why to reverse the field wires:

""All the field wires do is complete the circuit, so it shouldn't matter if you reverse them. It really shouldn't do anything different, but I've seen it work in cases like these. It could be a sign that the diodes are starting to go out, but I've also seen this work, and work for years.""

I have had more discussions with them and since the machine at NAPA said to reverse the field wires, he thinks that the diodes may be the issue even though the machine showed the alternator to be in perfect condition after the swap. I am going to swap the field wires and see if it makes a difference at idle. If it does, Powermaster will take it in for repair and return it to me as new condition. Even if swapping the wires does not work, I will probably send it to them anyway due to the NAPA machine test and I am not out any driving time due to our Winter weather.



Thanks