Show me the science.
I've already explained to you why!
When you heat up (or cook) a vegetable that contains water or in this instance a puree of onion, garlic, ginger, tomato puree, spices, oil, water etc., it releases all that water that you refer to and initially creates an emulsion (a mixture of all those elements, water and oil) in the pan so you don't see them as separate elements. The water evaporates as vapour and steam as you heat and cook it.
When you do a base sauce (or any sauce) reduction, what do you think comes out of the sauce and where do you think it goes? Or are you going to contend that too? This is empirical, we know it reduces significantly from the volume we start with and we know that it's water that's evaporating from it.
When enough, (usually most) of the water has evaporated from the mixture the initial oil/water emulsion created breaks (because there's now not enough water to hold it together) and the oil separates from the now virtually waterless mixture.
If you're going to contend this, I'm looking forward to your explanation of exactly
what disappears from a sauce when we reduce it and where this
thing actually goes to!
How about I don't until you explain why, when we know the vast bulk (80-90%) of the contents of the pan are water
Because it's not 80-90% and water only evaporates when it's heated. Your maths are FUBAR'd.
Over to you, for your explanation.