I only have one Muddy Waters album, At Newport 1960. Encoded as ALAC, the first nine tracks have a typical bit-rate, from about 800 to 1100kbps. The last four tracks, however, are at about half that. The Get Info window shows them as being stereo, but they sound mono. (The first nine tracks have some obvious stereo effects.) So I suspect that when both channels are the same, even if it's a "stereo" track, that the encoder is smart enough to know this and not duplicate them.
But even if that's the case, 260kbps still seems low. Is there a lot of silence in each track or something like that?