From the FAQ, it sounds like they simply presume that anywhere with multiple low NIC values is indicative of interference.
> The GPS interference data is derived from NIC (navigation integrity category) values that we receive as part of the ADS-B protocol. We mark regions as affected if a significant number of flights in that area report lowered NIC values.
> The GPS interference data is derived from NIC (navigation integrity category) values that we receive as part of the ADS-B protocol. We mark regions as affected if a significant number of flights in that area report lowered NIC values.