I know maintainers of projects have been hired directly by companies using their code as it is the most expedient way forward. Others might just offer up enough money to get the maintainer to take up a few of their specific issues/requests in a way that makes it worth their while. Just because someone is working on a project that is open source does not mean that money cannot be involved in the development. The company paying that money knows that the updates released as a normal part of the project will be available to anyone else using it as well.