Just out of curiosity(if you even know), why are the companies so all over the board on the WRX? From $47-$152/mo? Were those all the same coverage levels and such? That a lot of change there. The GTI is at least close(though I'd expect that with only liability I'm sure).
I'm not too insurance savvy, the last change I had to make was about 5yrs ago and I've always used my parent's agent. I'm paying about $50/mo. for my GTI currently with full coverage, dropped over $100/mo once I turned 25 
Ok, so basically here is the process. An insurance company has to submit how they want to rate policies to the department of insurance. The DOI then approves it, or tells them that certain aspects must be modified or removed, etc etc. I.E. if a company tried to say they wanted to use the driver's hair color as a rating factor, the DOI would probably not allow that. Some things are going to be consistent across the board for all insurance companies, i.e. If you have a speeding ticket it will increase your premiums. But once you get outside of the major stuff, every company will rate policies differently. For example, progressive is one of the few companies that people over the age of 65 will see increases in their premium due to age. Many companies don't increase premiums when a person gets older, for whatever reason they deem necessary.
The WRX issue is difficult to explain. You can see rates for certain cars be higher because of the claims experience that a company has with that specific vehicle. For example, the WRX was the first car that has a price/perf ratio that it did, that meant that a lot of young, inexperienced drivers were able to purchase these cars, and are consequently either not able to handle the power, or not able to handle it responsibly (think of what happens when you put a 17 year old boy in a fast car when he has some girl in the car he is trying to impress for example). Consequently that means that the rates for that specific vehicle are going to end up higher because the company will see a higher frequency of accidents or a higher severity of accidents.
This is the reason why you will see cars like RS4's and M5's and 911's end up being relatively cheap to insure. Generally the type of people that can afford a car like that aren't going to be driving recklessly and getting into as many accidents.
So, with that being said, different companies are going to have had different experiences with vehicles, which can account for the rate differences. I.e. if you have a company that predominantly carries older drivers with little to accidents (think when company like allstate will nonrenew your policy if you have say 2 at fault accidents), what's referred to as low risk drivers. Because certain people tend to buy certain cars, that company may not have insured many people who had WRX's, and if they did, because of the fact that they only insure low risk drivers, then those people who did purchase cars like a WRX were likely much more responsible with them and as a result got into less accidents, or less severe accidents.
I'll check up later if there are further questions.
Edit: I also want to clarify, because this comes up constantly with my customers. Although the value/repair costs of the vehicle do play into the rates, they are ultimately a small portion of what drives the cost of a specific vehicle. A very VERY large portion of the rate has to do with how safe the vehicle is when involved in accidents. Overall the majority of claims payouts come from medical expenses. This is why you can see in every state that the Property Damage portion of the state minimums is so much lower than the others. I'll cite CO as an example. They require a minimum of 25/50/15. That means 25k per person in Bodily Injury (medical expenses for the other occupants of the vehicle YOU struck, assuming you are at fault), 50K maximum per accident in Bodily Injury, and 15k in property damage.
It's very easy to have an accident that caused 9k in damage to a 20k vehicle, but caused 50k in medical bills. This is why our lovely little german cars, especially audi's/BMW/Merc's tend to have relatively low rates, they tend to be on the forefront of safety technology. Mercedes Benz especially.