That sounds exactly as expected...Your runoff had pH of 7, you watered with pH of 6, you then ended up having runoff with pH of 6.5 (the average of the two). Any kind of pure water should be right around 7, so not sure why yours would be 6. Is it a paper test kit? Or electronic pH tester? Maybe by a different test kit if it's paper...if it's electronic, make sure it's calibrated correctly. You could buy a "buffer" solution from your local grow shop and try your test kit out on that...if it reads anything other than 7, you'll know you have a bunk test kit.