Those plants look extremely happy with whatever is going on in the rootzone. I wouldn't worry too much about runoff readings. The only way to actually know what is going on in the rootzone is to catch the runoff of pure RO filtered water through the rootzone. If your measuring after a feeding, of course EC will be higher of runoff because your adding in fertilizer on top of what is being washed out.
In short, this explains why your readings would be higher, also your never going to get a 100% accurate reading because the coco itself binds to a lot of elements which won't allow them to be released into runoff to begin with.
Generated by ChatGPT
"Nutrients absorbed by coco coir are released back to the root zone through a process called cation exchange. Coco coir has a high cation exchange capacity (CEC), which means it can attract and hold positively charged ions like calcium, magnesium, and potassium. When plant roots come into contact with the coco coir, they release hydrogen ions (H+) into the surrounding medium. These hydrogen ions displace the positively charged ions attached to the coco coir particles, effectively exchanging them for the hydrogen ions. As a result, the absorbed nutrients are released from the coco coir and become available for uptake by the plant roots. This process is ongoing and dynamic, allowing plants to access nutrients as needed for healthy growth.