it seems there are few possible explanations :
1/ GPRS is occupying the rest of the available capacity
2/ not all timeslots are defined for TCH (?)
3/ you are not looking at the correct indicator :
a) maybe 11 erlangs correspond to the amount of “full rate” traffic, not “HR” traffic.
b) you are looking at the daily average traffic, not the busy hour traffic ? whereas TCH congestion % = 15% is at the busy hour.
just giving some leads..