mob drop bug

i founde a bug in mob drop 

i was playing minecraft and mining  and thin a skeleton killed me and he took my armor and my sword for some reason he took the sword and he was having a bow 

and when i killed him he droped my items + full dirabilty bow 

i think thats a bug or i was super lucky and i dont think thats luck becouse it cant be a weapon or tool drop full dirabilty 

(sorry for wrong selling) 

i hope that was helpful