Keef
The problem is that all the firms advertising in the GA press are brokers, some one-man part-time operations, and half of them are going via another broker which is usually Hayward Aviation (which you can go to direct if you have not already prevented yourself from doing so).
To get more than one quote, the customer needs to directly ask the broker which underwriter he (the broker) uses, and ensure that the same underwriter does not end up doing more than one quote.