I have never been able to figure out how insurance companies come up with their quotes. At the end of the day the only way you will know you are getting the best deal is to get on the phone to as many of them as you can and shop around. Don’t be afraid of telling them you have a better offer somewhere else, they may suddenly be able to find another discount option.
The only other thing I can add that has not been addressed by the others is to weigh up the reputation of the company as best you can (hard to do but you can get some idea by the number of complaints online). If you are ever in the bad position where you need to make a claim the last thing you need is to be dicked around by your insurance company. I will put in a plug for USAA here. If you are ever in a position to use them (they have restrictions) they are the best company to deal with. I have had claims with them 3 or 4 times and it has been a pleasure each time.