Where do Ins Compaies get info?

I hear tell that you never stay with the same insurance co. for more than 2 yrs. at a time. Over the years I have noticed that my rate goes up a little bit every year, like 25 to 50 bucks. When I change carriers the rate goes back to a figure the same or below what the previous policy was originally, change of vehicles not withstanding, with the same coverage limits. I started changing my insurance carrier every 2-3 yrs about 20 yrs ago after getting fed up with the rate raising in spite of no claims BS and think its been good practice. I have had very few claims over the years with no problems what so ever doing this.