But for some reason, people decided 1 DT = 6.6 PD here and most of people dealt with that rate.
Maybe because it was easier to remember that 15 DT = 99 PD instead of 99 PD = 12.857142857142857 DT (since DT were more likely to be traded for a whole stack of PD).
That's why you rarely see Spheres being traded, it's a waste to buy them.
Now server is poor on PDs so it varies a lot, some even trade at 1:5.