''Soul" Ron is I think a convenient euphemism - which may be better described perhaps as simply ''character''! Certainly just something perceived by the owner.
Older pieces, and certainly ones with history can I think be imbued with this - if only because we know that the gun has been in perhaps many hands and travelled etc. It has ''matured''!
Very new and modern guns with less than pretty appearance - in particular when bought new, have not had time to soak up any history!