channel separation

Have been doing some preliminary research about possibly moving from a nice integrated to separates.  Was hoping someone could set me straight on a measurement characteristic that has always bugged me:  channel separation.  Just read the Stereophile review of the Prima Luna EVO 400 preamp, which is described as dual mono from end-to-end. Yet, channel separation was measured at only 50 db (at 20 kHz) due to "capacitive coupling."

How can the channels bleed into one another when the entire layout is dual mono?  Why do solid state preamps seem to measure better in this area - is the output device of importance? Most importantly - does this matter? I always imagined that greater separation would mean greater preservation of image and soundstage.  Thanks very much!