On a number of modern receivers, the analog two channel inputs lead to an ADC and the signal is processed digitally from there. This is to apply various speaker and room corrections, as well as any DSP modes for that audio (be it standard processors that translate to surround, e.g. Pro-logic, DTS Neo, etc, or special effects that the receivers support like Yamahas are known for).
On some high end receivers, they also have a pure analog path where none of that happens. Depending on the model, speaker level corrections might take place, or they may not.