I listened to Craigs talks on pure water and its properties and had the properties demonstrated so I am assuming some things.....commonsense application of things told and observed.
I would assume that DWR is a function of surface tension of water in conjunction with the porosity of the fabric upon which it is deposited and any contamination of the fabric. For example: If you drop water onto a plastic bag it will sit there unitl it evaporates and if you drop water onto a netting fabric it will drop through to whatever is underneath. Breathable garments being somewhere in between those two extremes - needing to be porous enough to allow internal moisture out, but also with small enough pores to keep moisture out.
Because water needs contamination (dirt) to stay stable any grub on the fabric will encourage the nice clean purified rain to latch onto it, therefore soaking more readily into the fabric.
One of the demo's I watched to show how clean filtered/purified water is in comparison to tap water is to put a few drops of each on a perfectly clean sheet of mirrored glass. The water evaporates and the tap water leaves behind tiny, but visible, rings, wheras the purified water leaves behind no marks whatsoever.
So if the clothing is clean and has a functioning DWR the rain will sit on it until it evaporates - not allowing for the Brecon or Dartmoor levels of rain here. Any contaminants and the water will find them and latch on soaking into the fabric.