Because 3 erlangs means that in average 3 TS are busy. But due to random arrival of calls in the cell, 3 erlangs means actually more than 2% of calls are rejected.
7 erlangs means a pure perfect allocation of TCH, but people are not perfect. They might call at the same time, and not wait that someone else free the TCH.
This is Erlang B law… statistical occupancy…