Before equal temperament flattened everything, keyboards were tuned so that some keys rang pure and others had character — and traditions outside Europe never used twelve equal steps at all. Pick a tuning to see its degrees in cents, play it, and (for the twelve-note keyboard temperaments) hear the same major triad shift colour as you move it around the keys.
In an unequal temperament a major triad sounds sweeter in some keys than others. Pick a root and compare the tuning against equal temperament.
Standard MIDI is locked to 12-ET, so the plain .mid is the nearest-note approximation.
The MPE file pitch-bends each note to the exact tuning for an MPE-aware instrument.