With each AMD graphics driver taking about 11 weeks from start to finish, in order to provide driver updates on a monthly basis we have the AMD release train. At any moment there are at least two driver releases under development. When one driver leaves development and steams forward into the validation process, the next driver enters development, and the process repeats itself. For instance, in May we saw the release of the 8.37.6 fglrx driver and development for next month's 8.38 driver has already ended and is in the validation stage currently but will be entering beta shortly. Meanwhile, development on the July 8.39 driver will be underway shortly.
If you're a FireGL owner, you may have noticed that the unified Linux driver on the FireGL series page is not updated on a monthly basis. According to Matthew Tippett as for the reason why it's not updated monthly, "Releases with a non-monthly cadence typically have considerably higher levels of targeted testing. In particular for the Workstation driver, there is more focus on the Workstation ISVs and certifications." For features that take longer than a month to implement, Tippett had went on to explain the following: "As expected high risk or experimental changes would be developed in separate trees, but long term features, architectural changes all happen while the release train is continuing. The analogy I like to use is that we are on the train, and to add a new carriage or update the carriage, we have to do it while the train is running, without stopping the train, or letting anything fall off."