I almost never have a level horizon. Usually always off about 1 degree (usually must rotate counterclockwise to correct). I have pondered this and some thoughts:
1) my head or eyes or brain is tilted
2) I rotate the camera slightly when I press the shutter
3) maybe the sensor is rotated slightly in relation to the viewfinder
4) you just can't consistently be perfect
It cannot possibly be number 4. And it therefore cannot be 1 or 2 either.
This means the reason must be number 3.
What perfect logic.