Furman polarity?

Well, BALANCED is 60 volt hot and 60 volt neutral.
So the actual result would be 120 between the two, or 60 hot to ground, and 60 volts neutral to ground.
If you are using some inexpensive 'checker' it make just not work on balanced outlets.
ALL USA "BALANCED" power is 60 volts/60 volts.

The issues of balanced power are complicated enough that it is illegal to have it in the walls of a residence per codes.
In industrial uses, it has to be clearly marked as balanced on every outlet.

Both of my power conditioners are outputting balanced A/C.

As does my isolation transformer...used only for low current or digital gear.
It is a 400va transformer.

Interesting question, I use a P-2400 IT, but have never made any attempt at comparing it's output to input. I would have assumed that an isolation transformer effectively removes any relationship between the AC input hot/neutral and the transformer output. I guess one could measure the AC phase of the output and match it to the input, but that would seem to be an arbitrary relationship anyway.

..and I'd actually expect that most simply line testers would at least show some kind of neutral/earth fault when measuring balanced AC output.