From my understanding, the new CT machines are able to characterise material composition using dual-energy X-ray, and this is how they were able to relax the rules.
At an elemental level, the materials of a suitcase are more or less identical to an explosive. You won’t easily be able to tell them apart with an x-ray. This is analogous to why x-ray assays of mining ores can’t tell you what the mineral is, only the elements that are in the minerals.
FWIW, I once went through an airport in my travels that took an infrared spectra of everyone’s water! They never said that, I recognized the equipment. I forget where, I was just impressed that the process was scientifically rigorous. That would immediately identify anything weird that was passed off as water.
[1] https://en.wikipedia.org/wiki/Spectral_imaging_(radiography)
https://en.wikipedia.org/wiki/Dual-energy_X-ray_absorptiomet...
If your liquid is 80%+ water (that covers all juices and soft drinks), it is not going to be an explosive, too much thermal ballast.
Something like 10 years ago, I had my water checked in a specialised "bottle of water checker" equipment in Japan. I had to put my bottle there, it took a second and that was it. I have been wondering why this isn't more common ever since :-).
No idea if it was an "infrared spectra machine" of course.
It's a good thing that airport brought some machinery to apply the rule in a sane way. But it's still an insane rule, and if it wasn't the US insisting on it, the entire world would just laugh it off.