Different pH & PPM levels with every runoff !?

Can someone please help me understand this. I am growing in 50/50 coco & perlite using General Hydroponics Grow and Bloom for coco. I am currently in Veg feeding my plants 600ppm and feeding every 2nd day(one day feed, one day nothing, one day feed, one day nothing etc..). My plants are generally in good health a few purple stems here and there but overall good.

The thing that is bugging me is when I feed I'll hand water with 600ppm of nutrient solution with a pH of 6.0. Then the very very 1st runoff that comes out I'll catch to take a measurement and it will show me around 500ppm with pH of 6.2. Then I will continue throw a little bit more of the nutrient solution and take another runoff sample after a few minutes. Now all of a sudden it shows like 300ppm with a pH of 7.0.

When I feed I just throw in till it starts to runoff then stop but I know a lot of people say u must keep throwing till the runoff is the same ppm as the solution. I have tried to keep throwing liters and liters and liters of solution through till it eventually shows 600ppm at the runoff but then the next day it will be the same exact story 1st runoff is 500ppm then a little later it's 300ppm.

Can someone please explain what is happening here

Buffers In the growing Media, its all good don't sweet it. PH swings are good in fact its beneficial , stop testing run off.

you cant fight growing media buffers, and you shouldn't.

feed what you feed and unless plants show signs of being unhappy just roll with it. Ph swings in growing media thats what makes all nutrients available, Wet /dry affects ph, thats good, you want a PH swing.

plants need variables, they need fluctuations in PH and ec/cf

stop testing and learn to read your plants, focus on the new growth, it is good keep doing what your doing.