I work with insurance in the US. Retention on monthly pay is significantly better and has been for every company I've worked with. Also, monthly pay customers often pay more for insurance. The insurance company makes money by holding the premium cash between purchase and claim payment and keeping the returns. This is more or less a leveraged investment purchase. If you pay monthly, the company will make you pay more. This is most often done by providing a pay-in-full discount, but some companies also add transaction fees per payment and other misc. fees.
I have no context to say if the amount listed is a reasonable average, but as the other poster mentioned, new drivers or high risk drivers will certainly pay very large premiums that will tend to skew the average higher than the median.