To be honest with you I barely ever check run off,unless duty calls I think its pointless i keep my water at 6.5 all the time when it goes through the soil, im not gonna lie have never really had much of a problem with ph in soil it stays pretty stable.ok now to the problem what have you ammended your soil with sometimes you just gotta give it a few good flushes and that run off will become more stable. I have learned that some soil additives such as roots organic will cause you to have a low run off in the 5s, im speaking from personal experience as long as you keep the water and nutrients at a 6.3-6.4 going in ur fine it will level out, I have never had a ph problem yet . Good luck hope i could help. Ps in my experience the droppers are alaways a little lower when they read next to a ph pen