If only they were “plucked out of the air” in reality the best acturial brains in the country (and often from abroad as well) combine any and all data they can get to generate a premium that’ll better reflect your risk than the next company. If anything the Insurance business is a death match between the best maths and marketing types we can muster.
So the premiums are based on a combination of risk factors, postcode, age, driving history, credit score, car, occupation, driving experience, the list goes on try combining 30 or so variables.
The bigger the company and hence the more data they hold and quality of analysts they employ the better they rate your risk, that doesnt mean the better premium you see in fact often the opposite as they are less likely to under estimate what you should pay but equally are more likely to correctly identify a better risk.
All this is secondary to your profitability which may be different from your risk so a safe prospect with some risk may not be as profitable as high risk priced correctly if you can attract enough of them to spread the load. This may also be subject to a pricing layer in the pricing model.
So motor insurance has actually become that complicated I suspect(know) many companies have lost control of every outcome from every risk factor which is what has probably happened to the op.
Soon (few years)we all have blackboxs fitted which may not sound too appealing at first but at least our premiums then will be based on individuals not a mathmatical guess at someone like you.
Cheers