Effective port minutes vs. erlang calc

I’ve got a strange issue I’m dealing with and would appreciate any insight…

In my data center, we have an effective port utilization of about 7% of total minutes, meaning a port is used for about 3,000 minutes per month. (This calc is arrived at by taking total minutes of traffic, and dividing by peak number of used ports).

If I run my traffic through a typical Erlang B calc, minutes, busy hour, etc., I find this is off by a factor of 2-3. So, for example, at 500,000 minutes/month, it looks like I’m using 165 ports…but the Erlang calc says I should only need about 75…

Any help appreciated…

Thanks!