I don't know about the PH, other than milk is not always the same. I have had milk at 6.8 but not 6.91.
I have made that recipe, and the pressing instructions are not hard enough. The curd does not knit in the center of the cheese. Get pressing instructions from a different book.
I made one and after it had aged I picked it up and it broke in half. The center curds were still little pieces, and it was bitter because I didn't salt it enough. I don't know if I salted it like the book though, but I did press it like the book.