Hot air stores more water inside itself, and cold air stores less.
As such, cold air that is completely full of water and can’t store any more is at 100% relative humidity, aka 100% saturation.
When that air warms up, it’s able to store more, and as such the relative humidity goes down, similar to getting a bigger bucket that has the same amount of water. It’s now less full.
Thing is, your body doesn’t care about how much water is actually in the air, it only cares about whether it can take/give water to the air. Hence relative humidity is the only thing that matters.
Just because you are bad at utilizing your brain doesn’t mean an AI would be bound to those restrictions. The brain is actually an incredibly powerful computer.