Kegs and Deadmans Plate (possible bug)

Hello folks!

I'm not sure if this is a bug or not but when I built deadman plate on gp i noticed that the on hit bonus would apply on parrrley (if you chose to shoot somebody in the face). However if you parrrley a keg the deadman on hit wouldn't proc. Yet barrels will take sheen/triforce procs. So i'm a bit puzzled if this is a glitch or not.

Thanks for the help!