Half-and-half has smaller fat globules than heavy cream. The gelatin acts like a glue, trapping air bubbles that would otherwise pop.
This is why your grandma’s “impossible whipped cream” trick works. She was sneaky with unflavored gelatin all along.
Fun fact: This whipped half-and-half holds its shape better on hot desserts. The gelatin makes it bulletproof against a warm bowl of berries.