what it comes out the RO at is dependent on many many factors! tap pressure and flow etc is the main one.
I would persume you have killed your resin by not disconnecting the DI bottle for the first 10 mins before filling your tank the first few liters could be as high as 80 100+ tds. Also bringing it down from 17 in normal running conditions is high ish so i would guess its all to do with the above.