The equally tempered scale is dirty

[MadBrain]

-64 cents is simply the lowest you can detune a note using the midi tuning scale standard (thus the lowest you can detune a scale note on most keyboards). Basically MIDI allows values from 0 to 127, but to be able to tune both upwards and downwards, you have to use some of your range for negative values, which is why they went with -64 to 63 cents. So this value has nothing to do with Arabic or Turkish tuning, it's simply selected to give you increments of 1 cents over a range of just a bit more than -50 cents to 50 cents.