Originally, the distinction was made between ɯ as a back unrounded vowel vs. ï as a "mixed" vowel. In 1932 (Copenhagen) there were substantial changes, indeed one could say that the old IPA was tossed out for something like the new IPA, and at that point ʉ ɨ were introduced for central vowels. At that point, diaresis was given its current status of marking "centralization", i.e. movement from the standard reference position to something closer to the center. So actually, ï is a centralized close unrounded front vowel, not a central vowel.