At 120 MHz, the wavelength is 2.5 meters. The 15 meter cable is six wavelengths long. I would expect the longer cable to radiate more. However, you coiled it which acts like a big inductor or choke to keep that from radiating. I would expect if you uncoiled that loop, it would increase in emissions.
I have also seen test cables built a lot better than production cables. It could also be how they grounded the shields at the connectors. A pigtail ground would really increase the radiation.