Unfortunately the CFBundleShortVersionString Key in the plist File in Blender.app/Contentents/Info.plist
states only the main Version, which is 2.79 and not later versions like 2.79a. This has been always the case.
We use at our faculty a software distribution system to deploy blender (and other software).
By this we can not automatically track which version is currently installed on the clients, so the update mechanism would occur.
Please consider in future versions updating the CFBundleShortVersionString Key to contain the full release value, which is the case for almost all other apps.