It all has to do with what insurance is. Insurance is the transference of risk. You pay a premium to the insurance company, it turn they agree to indemnify you, or put you back to the financial status you were previously at prior to a claim within the limits you have selected. In simple terms you hit someone and owe for damage to his car, his injuries, damage to you car, and your injuries. Assuming you have all the coverages needed at the limits needed the insurance company pays for all this so you are now at the same place financially you were prior to the accident. That being said the, money you pay in premium is for the possibility of a future claim, not claims already paid. This insurance company must predict how much money will be paid on average for the next policy term for a pool of risk. It's like selling a product without knowing how much it will cost you to produce until after you have already sold it. Knowing this, it's amazing insurance companies know how much to charge. Charge too much and you'll lose customers to more competitive companies. Charge too little and you loose the company. You have to be as accurate as possible. This is why no insurance company should want to charge too much or too little. Auto insurance is so dynamic that it is almost impossible to predict more than six months in the future how much will be paid in claims. Variables such as people moving from one place to another, explosions in popularity of more destructive vehicles like SUVs, changes in driving habits like driving while talking on cell phones, and changes in jury awards are all variables that usually cannot be accounted for until the last minute. Further hindering insurance companies is the fact most states mandate that any rate change, up or down, must be approved through a lengthy government bureaucracy. This will slow down the process of charging the appropriate rates for the risk when time is crucial. This is why you will find that most insurance companies will offer only six month policies for autos. If you think it's all about insurance companies wanting to charge more every six month, then why are home owner's policies usually a twelve month policy?
In short, the companies want to be able to adjust their rates.
That can work in your favor also. Consider: An insurance company in most states can only charge for a conviction (a ticket) for 36 months from the conviction date (the date you paid it, or went to court.) However, the insurance company is not required nor is it even expected to stop charging for a ticket in the middle of a policy period. So if you got a ticket on 12/1/02 you would expect the reduced rates to come around 12/1/05. However, if your policy renews on 11/15/05 then if you have an annual policy you won't actually see the reduced rates until 11/15/06.