MIDI software picked an arbitrary number to stick middle C on. They chose a 7-bit range (0-127), wanted to start on a C (0), and wanted to put middle C in the "middle". Under these constraints, 60 is the C closest to the middle, and if you start numbering from 0 like computer scientists do, you get C-5, perhaps. However, octave numbering is not part of the MIDI standard, and lots of MIDI programs assign middle C a different octave number.
C-4 might be chosen because it's the 4th C on a piano, or a number of other reasons. Other octave numberings might similarly have to do with an intended musical instrument. Some articles on wikipedia call C-4 "scientific pitch notation" but I think this is dubious and/or pretentious. I think some trackers might use a relatively high number because of a practical need to support large sample ranges.
DoNotWant wrote:
This creates a linear pitch space in which octaves have size 12, semitones (the distance between adjacent keys on the piano keyboard) have size 1, and A440 is assigned the number 69. Distance in this space corresponds to musical distance as measured in psychological experiments and understood by musicians. (An equal-tempered semitone is subdivided into 100 cents.) The system is flexible enough to include microtones not found on standard piano keyboards. For example, the pitch halfway between C (60) and C♯ (61) can be labeled 60.5."
All of this has nothing to do with octave numbering. The linear space of 12 tones to an octave doesn't care which octave it's in. If MIDI had put middle C at 48, or if it was called C-22 all of these properties would still be true.
I'm not aware of any reason to pick one octave numbering over another that isn't arbitrary. Maybe it gets inconvenient to notate if you have to go above 9, or below 0.
DoNotWant wrote:
Thank you! I managed to find a real answear on wikipedia instead.
My answer was real, even though it was glib. It's arbitrary, and not standardized, in exactly the same way endian is.