pH of runoff is not the same as soil pH. Soil is a complex matrix of anions and cations and has an exchange capacity for each. Since you are using a seed starting mix, the soil is not very saturated with nutrient salts. RO systems are not perfect and usually still contain 10-20 ppm of various mineral salts. As your water passes through the soil, the soil may grab a few ions from the water. Since the source water is so low in mineral content a small change can cause a large pH shift in the run off. If you are concerned with the seed starting mix you should take a fresh sample from the bag and perform a slurry test for better insight. In the meantime add a small amount of tap water to your RO and adjust pH to spec. Good luck!