Windows 8 screen shutting off too soon

Could be a lot of possible reasons, here are my guesses:

1. It could depend on your mouse, I've had mice that even while completely still on a table send a very minor cursor movement signal back to the PC. Try unplugging the mouse to see if that's the cause.

2. There are applications that actively prevent windows from turning the screen off, full screen video is the obvious one, there may be something else you have running, check your background applications.

This Microsoft Support article may explain this mystery:

The "Turn off the display" setting may change unexpectedly when you configure the "Put the computer to sleep" setting in Windows Vista :

the Turn off the display setting is automatically set to the next shortest value when you configure the Put the computer to sleep setting.

Catch 22: This means that whenever you set the Put the computer to sleep setting to Never, the Turn off the display setting is changed automatically by Windows to a shorter time-period.

Solution:

To work around this behavior, set the Turn off the display setting to the value that you want, and then click Save Changes.

and, of-course, do not touch Put the computer to sleep afterward.