Thanks for your note. Your choice of which traffic model to use depends upon the nature of the traffic. Extended Erlang B assumes random call arrivals. Neil Wilkinson does not; it assumes that the traffic is peaked. It is usually used to dimension trunks groups that carry overflow traffic from other trunk groups. Neil Wilkinson’s blocking figure is higher because it thinks the peak traffic (rather than the busy hour traffic) is higher than it probably is (becuase the traffic is more peaky).
The short answer is that you’re not comparing like for like. Unfortunately, we do not support the Wilkinson algorithm at present, although we hope to in the future.