Take a simple, but extreme, example. Say that the CYA level is 50 ppm and that half of the pool water evaporates. The CYA does not evaporate so you have the same absolute quantity of CYA in half the amount of water so its concentration is doubled so is now 100 ppm (i.e. the "parts" is the same absolute quantity of CYA but the "per million" is half as much water).
You now fill the pool with water that has no CYA in it. You end up with the same absolute quantity of CYA that you started with and now have the same total amount of water that you started with so the CYA concentration is back to 50 ppm.
An even more extreme example would be to evaporate ALL of the water from the pool. You would then have a pile of solid CYA (or equivalent, such as sodium cyanurate) sitting at the bottom of the pool. Then add water to where you started and you end up with exactly the same CYA concentration that you started with.
If your fill water contains any substance, then that gets added to the pool water. So the above analysis works for Calcium Hardness (CH), but whatever CH is in the fill water will get added to that in the pool water which is why the CH goes up in hot climates with lots of evaporation when the fill water has CH in it, even if the CH is at a lower concentration then the pool water itself. Same goes for Total Alkalinity (TA) as well.
Richard