bx_masterdesk is built on a chain of processors which emulate analog hardware. To make this emulation more accurately behave like real analog processing units, we are using our TMT technology. TMT will cause subtle differences in channels, which is why you can see the gain reduction meter behave differently for left and right even if they are fed with the same signal.
When switching through the different TMT Compressor Modes you will see changes.

It seems the TMT variations we picked incidentally seem to cause a slightly higher level on the left channel of the Gain Reduction meter.
Summing up, we regard this to be expected behavior.