i tried hitting it in the center and the sides. it happens regardless, so it isn't from the keycap being unstabilized. my best guess is it comes from under the keyboard, like the keyboard is hollow under the left shift. this matches what air tree said about each key sounding slightly different, which they do, it's just the left shift is so much different
but what i really was concerned about is whether my keyboard is defective and i need to return it. it's comforting to hear that this is normal, and it's no big deal. i'm happy with this keyboard! i mean, i don't care about "fixing" the sound. i just want to leave the keyboard as it's intended