Because there are instances where those two speakers do produce different sounds. For example: when listening to stereo material in the 7-Channel Stereo mode, all the left speakers reproduce the left channel and all the right speakers reproduce the right channel. Also, when listening to a 5.1 source over 7 speakers and not using any of the back channel extraction decoders (EX, ES, CES), the side and rear speakers are run in parallel. The right surround channel is sent to the right side & right rear speakers; ditto the left side.

There are also situations where the two rears are not in similar acoustic environments; one may be in a corner that boosts its output, while the other is not. This imbalance can be evened out by the 950's ability to adjust volume on a per speaker basis.

Best,
Sanjay