Here's why:
* "Nut" and "hut" have a "short u" sound, but it's slightly different: The vowel sound in these words is closer to the "uh" sound in words like "up" or "but".
* Short "u" vs. "uh": The short "u" sound is a more closed, rounded sound. The "uh" sound is a more open, unrounded sound.
So, while "nut" and "hut" are close to the short "u" sound, they technically have a slightly different, more open vowel sound.