Due to Secunia's latest report Apple is getting a lot of bad press for being the most vulnerable vendor. I believe this can partially be explained due to Apple not updating QuickTime on Windows 8 & 10.
Update 2015-10-31: My original theory that the problem was with Apple's updating mechanism was incorrect. Apple have responded to my security report that QuickTime 7.7.8 is only available for Windows Vista and 7. It looks like you'll have to manually install QuickTime 7.7.8 if you're on the latest versions of Windows.
What's the story (morning glory)?
I originally was alerted to this error through Secunia PSI running on a Windows 10 device. Secunia identified that 7.7.7 had vulnerabilities and an update was available. I tried updating through QuickTime itself (Help -> Update Existing Software) however it advised me that I was already "up-to-date". I also tried the Apple Software Updater, but unfortunately this too showed that I was "up to date".
It appears the different versions of Windows respond in different ways with QuickTime 7.7.6 and 7.7.7.
Windows 7 with QuickTime 7.7.6 installed both QuickTime and the Apple Software Update tool identified that updates were available. However on Windows 8 QuickTime advised me that I was "up-to-date", whilst the Apple Software Update advised me that I need to update to 7.7.8.
Windows 7 and Windows 8 with QuickTime 7.7.7 installed QuickTime advised I was "up-to-date" whilst the Apple Software Updater identified that updates to 7.7.8 were available.
It appears that I'm not alone as there's a multi-page post on Apple's discussion forum. Like others when I tried manually downloading and installing the QuickTime 7.7.8 update it would fail with an error about the operating system being incorrect.
I've dropped Apple's Product Security team an email as it appears there's something wrong with the updater logic on Windows 10 (this is potentially true for Windows 8 and 8.1 as well as the Apple Websites only states the update is for Windows Vista and 7).
Update: Apple have responded with the following:
Thank you for contacting Apple Product Security.
QuickTime 7.7.8 requires Windows 7 or Windows Vista.
I've responded to Apple asking if they plan to release 7.7.8 for newer Operating Systems and will await their response.
I've also contacted Secunia to see if they're able to provide any breakdown on unpatched QuickTime vs. their Operating System. Will await their response.
The update can be downloaded directly from Apple's website. A few weeks ago this wouldn't install for me, stating something about only working on Windows 7 or Vista, however it appears something has changed and it is installing correctly now on Windows 7, 8, 8.1 and 10.