The trim settings in the receiver vary how much of the voltage coming in from the source material is let through to be preamplified and amplified to each speaker(the receiver "volume" control varies this uniformly to all speakers). For a more sensitive speaker, or one closer to the listening position, or whatever else might make it louder, the trim reduces the incoming voltage in its channel enough so that after the uniform gain that applies to all channels takes effect, it's no longer louder than the other speakers. This takes place at all "volumes", i.e., all levels of voltages coming in from the source material. Since the difference in sensitivities doesn't change with volume, there's no "matching" problem at different volumes.
