Not to worry..typical results..
The engine block and heater core hold the remaining amount you can't get out...What I usually do..is drive around a few hours or days and drain it again..the dilution rate is better the second time around...
BTW I hope you are using DISTILLED water
if your are using the Prestone Dex-Cool concentrate, instead of straight tap water...you don't have to worry about it though if you buy the premixed
50/50 Dex-Cool..just dump it in.