When killing mobs, epsecially with Knockback swords or Punch bows, the item drops will spawn where the mobs was before it died, but the XP will spawn where its "body" lands, several meters back. For example, reference my screenshot:

I was standing to the right of this image, facing towards the left of the image. I insta-killed two creepers with my Power V Punch II Infinity I Flame I Unbreaking III bow. I marked in the image where the creepers actually died (note the presence of gunpowder at both) and that the XP orbs spawned ~9 m behind the item drops, where the creeper's body landed after he got punched back.