iOS 9 beta 6

You're completely wrong. iOS 2, 5, and 7 all had a beta 6 that was not the GM. iOS 3 and 8 has the sixth release as their GM.

The real situation is this- for June and July, Apple develops their next major release via two forks- the beta fork and the secret fork. The secret fork includes support for the unreleased hardware. The last few secrets have been Siri for iOS 5, 4" support for iOS 6, Touch ID support for iOS 7, and large screen support for iOS 8. Now come August, Apple must begin the work of integrating the secret fork into the beta fork to create one release build. From the start of the integration until the iPhone September event, Apple is unable to release betas without disclosing secrets. For that reason, there are never any betas during that time. This takes about a month- 34 days for iOS 5, 37 days for iOS 6, 26 days for iOS 7, and 36 days for iOS 8. Therefore, you will not see anymore betas until the September event.

Does this approach have a cost? Absolutely. By stopping all seeding for a month, Apple does not allow sufficient testing of the release and near-release builds. This is why there are pretty much always battery life, WiFi, and performance problems when iOS goes public in late September that eventually get fixed towards Christmas. However, secrecy is absolutely essential to Apple's marketing strategy and must be maintained, even if quality suffers. That's just the way it is.

Hope this clears things up!