i think is something pretty simple to understand without going too deep into science:
If you get a 500 ppms solution into a substrate that has salt buildup in it, some of these salts will be found in excess in the runoff, getting it above 500 ppms.
if the plants drinks more nutes than water, the ppms in the medium will get lower and the next time you water with 500 ppms, your runoff will be a mix of the feeding solution and the salt stripped h20 that was in the medium, getting a lower runoff than imput.
That’s how you understand how much your plant is feeding if ratios are ok and the girl looks happy