Man oh man do I feel vindicated! Just got an email from fox farms about your ph runoff readings, and I was right! I'm not one to gloat but...... Alright I won't! Here is the email.
Thank you for contacting FoxFarm with your inquiries, I'd be glad to help. This value for your run off sounds totally normal. When you send a pH adjusted solution through your growing medium, exchange rates and solubility of certain constituents all vary. An acidic solution will be able to dissolve other acidic ingredients, as well as your medium will absorb many of the ingredients that are more alkaline, leaving your run off more acidic coming out than going in. Observation of your plants is your best indicator for growing healthy plants, but if you would like directions for performing a soil slurry test, I will send it to you. This is a more accurate way of determining the pH value of your soil, rather than using run off values. I hope this helps.