@sixwatergrog said:

If you can assume - as you said in a comment - that all "minors" are Dorian and all "majors" are Mixolydian then I guess that clears it up, but it also raises some other questions. For example, when you see a MODIII jam like the 8/1 Tweezer going from what you call Amin to Cmaj it seems more plausible that this would be a move from Aeolian to Ionian (relatives), and not from Dorian to Mixolydian since C-Mixolydian (C-D-E-F-G-A-Bb) would use a different set of notes than A-Dorian (A-B-C-D-E-F#-G).

But here's the issue: you're conflating key with mode. One is a harmonic framework (key), and the other is a melodic framework (mode). For instance, when we say "jamming in C major," what we really mean is that you're playing scales that work in the context of C major. There are a number of different scales that do: C major, C pentatonic, A minor, G mixolydian. When we say "this jam is A mixolydian," that doesn't mean that there's a "key" of A mixolydian. It means that Trey/Page are playing A mixolydian scales over a given harmony, often A major (which Mike and Page outline with their bass and chords).

So while it's true that when Phish "jams" in A minor they are usually playing A dorian modes, it's still important to remember that we can't just exchange key and mode freely.