A bell curve simply measures your median level of climbing ability in your gym. The "science" behind it is simply to cater to your customers, you need to set routes to attract the biggest demographic to your gym, which for most gyms is in the easy to intermediate range.
At my local wall, one of the ways we would determine the "median" of our climbing demographic was during our Everest Challenge event. Hundreds of people would climb day and night to climb the elevation of Everest on the wall, and track which ropes they were climbing, and how many times they climbed them. At the end of the event it was easy for us to look at the data to see which ropes were climbed the most, and determine which grade most people were climbing. The route setters would then know which grade of routes they needed to set more of.
I set routes for years at my university gym and for competitions. I can tell you it's easy to set easy routes, and easy to set really hard routes, but to set intermediate routes inside that curve at the level where the majority of your climbers abilities are - is quite difficult.
I'll give you an example:
Easy route = Bunch of big jugs close together in a ladder up the wall, there you go kids.
Hard route = A few garbage holds that are hard/hurt to grab and move off of; or, a series of really strenuous moves that the average person can't piece together without lots of core strength, training, and a lot of screaming.
Intermediate route (your bell curve routes) = trying to balance between easy and hard, without giving the stronger climbers the ability to dyno through half your route, bypassing many of the holds and ultimately reducing the difficulty rating of the route.
The ultimate challenge in setting intermediate routes lies in forcing beta. It's almost like a chess game, where you're trying to plan ahead in order to force your opponent to make the moves you want them to make. Forcing moves in a climbing route at an intermediate level is the hardest part of route setting, and takes a lot of experience, skill, and imagination. I would spend hours working on a single move in some routes, making it almost impossible to pass that section without using the exact beta I had intended them to use. The move wouldn't be that complicated, but if you didn't use it your balance would get thrown off to the point you could keep a grip on your other holds. Ultimately though, someone always manages to find a different beta to bypass a section, even if the move is more strenuous than the prescribed beta, and causes them injury in the process.
One thing I would do to force people to think about their footwork is use garbage holds like two-finger pocket holds in places where stronger climbers would normally cut their feet and campus through cruxes. I figured the threat of blowing a tendon would be incentive for them to change their climbing style, and actually spend some time figuring out the beta I had intended for them to use. Alas, instead of using their heads to solve the problem using a series of less strenuous moves, they would shove their fingers in the pockets, pull hard and shred flesh trying to dyno past half the holds.
The "curve" for each gym is going to vary place to place, and will be determined by the demographic of climbers that frequent the gym. Each gym may employ its own method for determining how many of each grades need to be set, some will simply be at the mercy of whomever sets the routes and the style of climbing they favour.