The main reason is the distance between exits. As a matter of fact, as Mr. Gaspar related to you earlier, even today, based on 1:40 figures, there are numerous cases in which you can demonstrate that you don't have a flight attendant for each exit. For example, the Airbus A320 and the Airbus A319 all have six exits. Under the 1:40 ratio, they don't really require six flight attendants. The Airbus A320 requires four flight attendants, and that's assuming you have a full aircraft, because that's based on passengers, not seats.
If you have percentages, as you mentioned, it would be less. Airbus A319, on a 1:40 ratio, has three, based on a seat capacity of 120.
The reality for the narrow versus wide body is the distance between exits and the fact that there are seats and galleys and so on blocking exits on a wide body, because they're in the middle and the exits are on the outsides, whereas on a narrow body, the flight attendants are in much closer proximity to those exits and therefore can handle more than one exit at a time.