This is because Apple has recently stopped signing for iOS 10.3.3 and iOS 11, which means that if you’ve already updated to iOS 11, you won’t be able to roll back to iOS 10.3.3 which was the previous update before iOS 11. For those who are still on iOS 10.3.3, this doesn’t really change anything for you, but note that once you update you’re stuck too.
For those a little confused as to why Apple stopped signing for iOS 11, it is because the latest version is iOS 11.0.2 which was released not too long ago with additional bug fixes and improvements. We expect that Apple will continue to update iOS 11 to improve upon the issues and bugs that users are complaining about, and iOS 11.1 has already been seeded to developers so presumably there will be some improvements in that update as well.