The most common device I've seen is carriers having subsidiaries. The regulation of insurance companies requires them to have consistent premium rates, subject to their evaluation of demographics, risks, history, etc., but whatever they come up with for their company has to be consistently applied to customers. They can get away with 'longevity discounts' (not sure about the reverse!) and few tweaks, and maybe a bit selectively applied. For broadly different rates, they'll use a subsidiary. When my son went away to college and was getting a car part way in, as a 20-year old, I was balancing on saving the dough and keeping him on my insurance. Then I decided, why not try one of those companies that claims to check everyone else's rates. I forget now who it was but it led to their affiliated insurance broker, Answers Financial, who got me Progressive at nearly 50% savings to all the quotes I had been getting, so I got him independent insurance. When I called Progressive directly, I got rates like all the other carriers- nearly double. What gives ? The Answers Financial broker coverage ended up being written Progressive Hawaii, who they must set up for higher risk, younger drivers and they offer, at least a few years back, much better rates. After about 1 year, "Progressive Hawaii"'s rates jacked a good bit and I was able to put him with a mainstream carrier for a decent rate. I never could confirm it, but I suspect that Progressive may have rewarded Answers Financial with an exclusive or semi-exclusive on writing the Progressive Hawaii policies because of the amount of business they do for Progressive.
By the way, I highly recommend this Answers Financial broker. They sell coverage for tons of carriers and can check them all for you. (Homeowner's too) My reps from these firms have typically been from west coast but I think their reps work from around the country. If you PM me, I can give you the current guy I've used who seems very good. My carrier for over 20 years has been Amica Mutual (great company all around if not the very cheapest.) When I got the Challenger, they were just a bit higher than I was willing to tolerate so my Answers Financial broker found me Plymouth Rock Assurance and I separated off that car only to them. (Amica said please don't do that (multiple policies for different cars I own), but admitted they can't prevent it)