The key to "ppm" is that it is a ratio of water to nutrients. If you have a rez of solution at a given ppm, and then add plain water to it, the ppm will go down as the concentration is less in more water. The same happens in reverse. If you lose JUST water from that solution, the ppm will go up because there is less water to nutrients (the ratio rises). If you take that same rez solution and extract some of the nutrients(and very little water) from it (like the plants or evaporation do) then the ppm(ratio of nutes to water) will fall.
What is happening sometimes is the plants are sucking up water faster than nutrients. This can be due to the plants being too warm, which causes them to "sweat"(transpire) more which causes them to take in more water. If your atmosphere is very dry, that can also make the plants drink more water.
Now there is another possibility to this cause. Depending on the nutrients you are using, you could have left over salts in the solution after the plants take in the nutes that are bonded to these salts. Salt is an electrical conductor so it will show up on a ppm measure as nutrient that are present. I get this a good bit with my rez.
To combat the salt build up that can cause you a lot of issues, I drain my rez every 7-10 days and completely replace it with new. The trees and yard grass love that I dump my rez in the yard. They have never been happier. I also (at the 3rd week of flower) will flush the whole system and plants with just straight water in rez for 12 hours, then dump that and replace with new solution. I do this once per run as this prevents toxicity in my system which can wipe out your plants.
What I do with mine is set my flowering rez around 1200ppm (for my nute brand) and watch it for several days. If the water goes down and the ppm doesn't, I add back straight water (or if the plants are in real high growth at the moment) I will add back the water with only half of the normal nute regimen. If the ppm rises to above 1400, I change the rez solution. I rarely see my ppm go down in my rez, unless the plants are really eating and drinking, and I add back straight water. Then I can see the ppm drop.
I hope all this helps more than confuses