A deer sure can jump the string at 321 fps, exspecially at closer to 30 yards. You probably hit higher than you think and hit the gap between the lungs and spine. Happened to me once. Father-in-law ended up shooting the buck during gun season and while skinning found an X scar on each side. Stuff happens I guess. I have personally taken 9 deer with a 4 blade 90 gr Muzzy and they have done well by me. Usually I am pulling the bloody arrow from deep in the ground.