It is not an up or down reading. It is the lenght of the green. Exemple: oakmont hole #11:
+17 on top -17 at bottom 34 between brackets (17 + 17 = 34)
the flag is at -11 from center, in that case it means 6 yards from the front fringe. The difference between -17 and -11 = 6 yards.
if flag would have been at +1 then it would have been close to center of green, since 0 is center.