Why is reservoir water climbing in PPM daily?

As your plant feeds. If you've too much in the way of nutrients. The ppm will go up. But, with a lower ppm the water / nutrient ratio should intake evenly enough that you to up your water and your nutrients at the same time. This is the ideal level (nutrient/water uptake) At the moment. Your levels are too high. And your plant is taking in more water. This can be down to an imbalance in nute/water mix. Or it could be your nutrients haven't got the right catalyst to help your plant absorb nutrients better. (If this is the case, you'll see light green at the edges of your leafs (too much water) But, with DWC when your ppm rises it gen means your nute mix is off. Try using a lower mix. And when the ppm.goes down you'll have your plant taking in more nutes than water. And by doing that. You'll find your right mix levels.

Evaporation, causing the solution to become more concentrated.

As already stated, a plant's feeding habits can do this....so can simple evaporation. This is part of the reason they typically suggest using half strength nutes when topping off your reservoir. If your plants are responding ok(no burn) I wouldn't mess too much with your nute levels. The plant might want to feed harder next week.

If the water level is decreasing, pH is stable, and ppm/EC is rising, it does indicate a need to lower ppm/EC. Opting to prioritize EC for precise monitoring over ppm is a wise decision. Viewing a 20 ppm increase as minor, your suggestion of adding 20 ml of water for detailed control and patiently waiting for changes in ppm/EC, ensuring pH and ppm/EC stay the same, signifies perfect conditions. closely tracking ppm can be a bit of a hassle😅😁

Yeah as Berry said. Your plants using more water then nutes. The solution probably didn't need to be as strong. If your Ppm was dropping your plant would be taking more nutes so your solution could be stronger.

Yes it is simple, you plant does consume a bit more water compare to nutrients inside the reservoir. So the amount of nutrients PPM (parts per million) raise compare to the amount of water. Next time feed with 100 ppm less and see if it will stay stable. Merry Christmas buddy! 💚