So I was curious about why the Atlas V rocket has so many versions, with three-digit designators like "401" and "551". The obvious differentiator is the number of solid rocket boosters you can add to the core rocket. Also, there's an option for extra engine for the second stage (though that hasn't flown yet AFAICT).
One thing I don't get though is how many different payload fairing options they offer. They have a 4-meter and a 5.4m fairing types, with very different designs, but both have 3 different length versions. And while, say, 5-m Long is a lot longer than 5-m Short (by almost 5 meters), the three different versions of the 4-meter fairing have almost negligible differences.
My question is why. Wouldn't it be better to just have maybe two versions, one standard and one extra long? Perhaps this is a stupid, opinion-based question. But is there an obviously good reason for this? Do the 4m and 5m fairing types feature a different design because of some important trade-off? Is the weight difference between the length options significant enough to warrant making them?