Remember the software does nothing particular. I.E. Your comment about it being clever enough to put Brian's guitar in the back at times. It isn't doing this on purpose. This is due to the way the stereo track was mixed. Obviously, to me, these particular sections you mention are being high-lighted for a reason in the original mix.
All the software can ever be doing is thus:
Center Channel = Phantom center of a mix with some bass cut to remove most bass guitar (there will always be background instrumentation like drums here too.
Front L+R = The original mix usually with the phase information taken out for...
Rear L+R = The phase information from the original mix now inserted into the rear speakers.
And there's even one for the sub, but not much really, unless we're talking Dolby Surround films.
Adam.
Adam Baboolal