I've experienced similar issues, and there are several possible causes:
- Mouse
Some mice, specially the gaming mice, are especially sensitive. Small vibration may cause them to be active, and wake up the system. It's also possible that when they become inactive after a period with no activity, they also wake up the system. To check this, you can power off the mouse, and see if it still wakes up.
- Notifications
On my KDE system, notifications will wake up the system. You can check this by checking whether there is a new notification whenever your monitor wakes up.
For me, the time to enter energy saving mode is controlled by xscreensaver. There are many possible things to control this, you may check your power settings in the system settings.