Let me give you a heads up about the insurance industry - premiums contain a random element. It's hard to go into details unless you have a detailed statistical background.
But in short, claims are a random distribution that is unknown to the insurance company - they simply try to estimate what the distribution is (even the shape is unknown) and then make best guesses for the parameters to predict the future experience. But as this changes over time, whatever form they use at any point in time is only an estimate, which changes as future experience comes to light.
What this means is that various policies get assigned to different distributions (meaning you get a different premium) and then the final outcome of these is compared to the projected experience. This helps to better predict outcomes in the future.
Therefore, you can get a different premium, even with the exact same risk at the exact same company. Different companies is a whole different story.
For example, comparing 2 different insurers:
- IP1 was $300 cheaper under insurer 1 compared to insurer 2
- IP2 (a bit different to IP1, but not by much) was about $300 cheaper under insurer 2 compared to insurer 1!
Long story short, shop around. Do NOT use the same insurer every time as I guarantee you the cheapest insurer will be almost certainly be different every time (and often by a significant margin).
In fact you can see this above already - different people have found different insurers to be cheaper on their properties.