Here's why:
* Long U sounds: The "u" in words like "flute," "music," and "rule" makes a long "oo" sound.
* Short U sounds: The "u" in words like "cup," "sun," and "duck" makes a short "uh" sound.
Therefore, the "u" in "fuel" is closer to the "oo" sound in "flute" than the "uh" sound in "cup."