Here's why:
* Stress: The stress in "fortune" falls on the second syllable, "tune".
* Unstressed vowels: Unstressed vowels often have a reduced pronunciation, sounding like the "uh" sound.
* "for" in "fortune": The "u" in "for" is not stressed, so it has the reduced "uh" sound.
Therefore, the unstressed vowel in "fortune" is the "u" in "for".