1st grow cookies gelato RQS

fed all plants and tested run off waters 1/31 gave all the plants the same feed 2500ml of water with half of the nutes recommended for a whole gallon 0.25 ml mammoth silica 0.5 ml cali-mag 2.5 ml F-Micro 0.5 ml F-Grow 1.5 ml F-Bloom starting pH - 7-7.5 starting ppm - 400 plant #1 runoff ph - 5 ppm - 1500 plant #2 runoff ph - below 5 ppm - 1700 plant #3 runoff ph - 5 ppm - 1390 plant #4 runoff(with nutes) ph - Below 5 ppm - 2500(hot) plant #5 runoff ph - 5 ppm 1290 plant #4 runoff (plain pH 7 water) ph - below 5 ppm - 3000!!!! why are all of my runoff ph so low even when I give them high ph water? im using a liquid ph tester, could it be inaccurate? switching to flower after next flush 10am-10pm will be giving all the plants a good flush next watering and testing ph/ppm again