Why so high? I try to keep it around 6
Because it's more optimal, especially in late flower. I just do what the plants tell me to do. Do whatever works for you. You have enough Calcium in the medium. Runoff pH doesn't tell you how much it may be lowered til next watering. From say 6.0 and then to 5.3 before being "corrected" by 6.0 but not enough.
The pH of the medium may be out of range, the runoff is just an indictator of what you should do. I feed at pH 5.3-5.5 because my pH keeps raising slightly on me in flower. The same goes if the pH were to go lower on me I adjust and add more carbonates "Calimagic" to the mix.
Ca is way more accessible at pH 6.3.