Total posts : 45366
The difference is the FCC OET test included the use of the AC ground. AC grounding is described in the patent information supplied for certification. The FCC accepted the fact that the AC ground, regardless of where it runs, is the system ground.
Measuring the added radiation from the “ground” conductor(s) of the a-c wiring of a building that are used to connect the chassis of a Part 15 AM transmitter to a non-radiating r-f ground of a Part 15 AM system (such as a buried ground rod) most likely was not included in the compliance testing of that system.
Probably those compliance tests were conducted in an anechoic chamber which volume would not permit the emulation of a system installed on the second floor of an apartment building.
Even if those tests were conducted at a “Open Air Test Site” (OATS), it would be highly unlikely that such a structure/installation would have been used there.
The FCC accepts/certifies a Part 15 product based on the test conditions and data they collect.
But if a buyer/installer/user of that product does not follow the installation configuration described in the FCC compliance test and certification, then it is reasonable to predict that s/he may be at increased risk for an FCC NOUO.