Here's why:
* Short vowel sounds are single sounds produced with the tongue in a relatively fixed position. Examples include the "a" in "cat" or the "i" in "bit."
* Diphthongs are sounds that glide from one vowel sound to another. The "i" in "grin" starts as a short "i" sound (like in "bit") and glides towards a "y" sound.
So, while "grin" has a short "i" *at the start*, it's not a short vowel sound throughout the whole syllable.