Seven speakers, hence seven channels. As Gonk says, right now that rear is mono, but it may not always be. The original ProLogic specified a mono surround channel (so it was actually a 4.0 surround mode) but ProLogic II does send a separate signal to each surround speaker.
This way you're ready with your amp; if you'd connected two speakers to one amp channel it would be difficult to send a separate signal to each later.
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Matthew J. Hill
matt@idsi.net