Thats an interesting statistic, but i'd like to add that I very rarely see John make a pass through the oppositions midfield into Lampard or Drogba etc. A lot of his passes go to his defensive partner, the left back or the player who protects the midfield, whether its Mikel or Essien, which leaves them to do the rest of the business. Thats a luxury the defenders can afford to have at Chelsea, so this isnt a slight on him or anything, whereas at Man Utd or Arsenal, you've got ball playing center backs, ala Ferdinand and Gallas, who can make the passes through the oppositions midfield. It allows us to play our attacking game and negates the need for a 'defensive' midfielder.
Thats just my opinion, but I personally see that as the reason why Terry has such a high percentage completion rate.