This is a consequence of Grin implementing the so-called Mimblewimble protocol. Quoting from [1],

> In Mimblewimble, outputs are Pedersen commitments r*G+v*H which combine value and blinding factor into a single curve point. The blinding factor serves both to hide the value and to control ownership. Correspondingly, a single (multi-)signature serves both to prove value balance (non-inflation) and to authorize transfer of ownership.

[1] https://np.reddit.com/r/CryptoTechnology/comments/kyhgcv/are...