There is a difference between stereo channels, configured via the global Show configuration, and channels linked via ganging.
Stereo channels are using stereo processing, so there is a stereo compressor with only one set of parameters for that stereo channel. The gain reduction will be based on both inputs but be applied also on both input signals.
This will be also be the case when you use ganging and link also the compressors (in fact you can only link blocks of parameters, not only a single parameter like ratio or threshold).
And that behavior makes imho the most sense. If the compressor of each side of the stereo signal acts independent from the other side you will get an unsteady sounding result.
