Why does my pH change so much in the resiviour but not run off

Drip system in coco/perlite

Last night I set the pH to 5.8 (didn't have enough to to make sure it stays that way...) came back 20 hours later, the pH is 6.2 (I was collecting it in a bucket) but in the resiviour it's 7.1. Why did it change so much in the resiviour then in the run off. All I have is the pump and a recirculating system in there. Can anyone explain this?

hi dude your coco buffers ph to a certain extent, so hence the ph of the run off being different to your tank.
the temp of the liquid you are testing can affect your readings, did you leave the run off sitting in the bucket maybe evaporation has changed the ph.
i shouldn't worry too much as long as your plants look well