I can tell you a little bit, but unfortunately some of the specific details and how they interact are beyond us! The Homplug communication standard (2 wire Ethernet) uses a series of 84 sub carriers in the frequency range from 4.5-21 Mhz. When dealing with multiple high frequency electrical signals, the sum behavior and how it varies with cable construction can be difficult to predict.
See here for some additional details I previously posted:
During past testing, we actually found that some specific cable constructions actually exhibit increased bandwidth past a certain point at longer lengths before decreasing again. We believe in those cases that we hit upon some multiple of the frequencies involved in some way.
-Adam