Hi,
The Mobile Carrier report is calculated by looking at a unique key derived from a combination of domain and ISP. Using this method, breakdowns of mobile traffic by known carriers and connection type are available to see WiFi by carrier traffic. The drawback from this method is that many carriers mask their mobile domains via mobile proxy. I believe the "Not distinctive" line item are those carriers.
The unknown line items on the other hand would be the ones where the javascript function was unable to fire.
Thanks!