Its processed surround though. The single sent out from the Wii is still stereo. If its Pro Logic II I can explain the steering to channels. Think of this as a table with two columns separated by the dash.
Audio out from wii - play on speaker
Sound is only on the right - right speaker
Sound only on the left - left speaker
Sound is on both left and right and exactly in phase (mono) - play in front center
Sound is on both left and right but opposite phase - play in rear speakers
Sound is on both left and right with opposite phase and the right is slightly louder - play in rear right
So it pretends to be surround but its not very good quality and depends heavily on your receiver being setup properly. It is also sent as an analog signal that is later amplified. Compared to a device with digital out this is a crappy solution. Even the Gamecube was capable of better audio