Well, let's think about it this way: in fourth gear which is 1 to 1 (meaning one engine turn equals one driveshaft turn) you would add 1 to the rear end ratio of (say for example 3.07) and this would give you an overall ratio of 4.07? How about in fifth gear where 86/100 of one engine turn equals 1 driveshaft turn? That is to say in fifth the engine is turning more slowly than the driveshaft at this point by about 14%. If we added .86 to 3.07, we would have 3.93. Seems like multiplying them makes more sense, doesn't it?
