Topic: Question about deviation between output ch, of RME Fireface UC

Hi, I'm using Fireface UC thesedays.

I found something wrong with Fireface UC.

I used the signal has A dB Ave. RMS and I checked how much dB loss happens when the signal through from output ch. to input ch. of Fireface UC.
(2 way 6.35 pi to 2 way 6.35 pi cable was used)

When I used output ch.1, 3, 4, the signal which was input to input ch. has A - B dB Ave.
But when I used output ch.2, the signal which was input to input ch. has A - B - 0.5 dB Ave.

Could it be happen as deviation between output channels? or something wrong with my Fireface UC?