I keep seeing the Left output of a stereo pair as louder than the right in totalmix

they are coming out of the same synth (Virus TI)
and when I connect headphones to the headphones output (headphones output receives the same mix as Output1+2 on the synth)
both sounds appear to be the same level.

but in total mix the left is clearly much louder (about a cm difference on the VU meters)
and I can hear the Left as sounding louder also when I connect headphones to the FFUC headphone output.

I have checked all Pans/Balance knobs in Totalmix are set to centre.

what really puzzles me
is when I swap the Jack leads around
the Left is still louder

(where as if it was an issue with the synth of the synth patch, swapping the leads would logically make the right channel in totalmix appear louder)

is there some kind of bug with my FFUC?
or a known issue or firmware update or something I should apply?

I've tried to think logically through the set up
and remove all other variables from the signal chain
as a work around I can eliminate this level difference by panning my synths slightly right.... but that seems like a silly fix
I'd rather fix the underlying cause.
I can take some screenshots or make a video if that helps with diagnosing the problem.

any ideas?
anyone ever encounter anything similar?