the art of shopping….. for car insurance

My son got his driver’s license a few weeks back and we got a car for him to drive.  with that, I had to get an insurance situated.  Our insurance company was quick to add him to the policy even without us requesting them to do so.  I filled out numerous amount of forms on-line to get a quote, but all were so high.  Then I thought ( and I know everybody else already knew, but I did not, late bloomer) if I get a policy on my name and add him as a driver.  Magic, insurance went down, but not as much as I was hopping.  Then agents started emailing me or calling with their numbers. Also I found out that some insurance companies will not insure separate, if you put a car, they want you to put all your household cars with them ( 21st Century insurance -fyi). Liberty Mutual got my business because they were half of the other’s.  One of the agents was very pleasant to work with and I asked her for a quote to match Liberty, and the closest she was able to do – the same amount, but for 6 months I don’t think so.  At this point we had 24 hours pass, and guess what, she just emailed to me that re-quote, and her new number $150 more than her 6 months , but it’s for the year.  How did that happened?  I probably will never understand what their quotes are based on completely, but such a difference – I can’t wrap my financial brain around it.