Reading here the x-ray machine model or type might be a true technical reason why there are seemingly always different rules. Even same airport, different terminal canbe different due to different build year and machines. What I don't understand is why people simply don't pay attention? By the time it's my turn I have seen the 3-10 people in front of me go through the motions, so I naturally preempt and prepare based on what I have observed. Not a guarantee for no issues but a solid step to not mess up completely.