I've worked extensively with the Aguila subsonic .22LR loads. Out of a full-length (i.e., 18") barrel the 40gr runs about 1035fps while the 60gr barely breaks 900fps. So both are soundly subsonic.
The 60gr is an unusual load in which the bullet is crimped in a .22 Short case. It's easy to recognize, in contrast to the 40gr which looks like most other .22LR ammo. (60gr is on the left in the following photo):
As James Jenkins guesses, the 60gr load uses more powder, and holding all else equal more powder produces more gas volume and "uncorking" pressure, and therefore more muzzle blast – i.e., more noise at the muzzle. For the following photo I pulled four different cartridges and weighed the powder charge. The Aguila 60gr has 0.9gr of powder, whereas both the Aguila 40gr subsonic and a CCI subsonic load have only 0.6gr of powder. (For further comparison, the supersonic CCI 40gr "Mini-Mag" has 0.8gr of powder.)