To my knowledge, any Mac that is 1. connected to power and 2. not powered down will wake periodically for the reasons I explained, and perhaps for additional reasons known only to Apple. If disabling "wake for network access" does not change anything, all I can say is that it's working as Apple intends it to work. Before you ask, I strongly discourage you from finding clever system "hacks" or workarounds to prevent your Mac from working as designed.
It may or may not be a bug. Whether it is or not though, if it bothers you, consider providing feedback to Apple.
When questions such as yours arise, it helps to look at iOS. iPads and iPhones will all periodically connect themselves for the purposes of downloading emails, Messages, alerts etc. There is no way to disable that, other than to completely power down the device, which almost no one does. Apple has been inexorably changing OS X to effectively implement iOS design features such as this – for better or worse is not for us to say.
May I ask why this concerns you? Perhaps that would lend some insight regarding possible solutions.