Something no one has noticed
      

  You gain bonus xp when you kill several enemies in rapid succession, or kill several enemies with one attack. It was probably the former, which would wait for a few seconds to see if you killed any more demons before granting the XP.This is very likely the explanation. It seems about the right amount of time before the combo XP is award..
      

http://www.blizzblues.com/us/something-no-one-has-noticed-27300876044.html