I hadn't heard this before, so I can only guess. If it is was intentionally included in the design, then it is most likely a mute of some sort applied to unused channels when in stereo mode to prevent unwanted interference or noise from reaching the amps. It could be considered similar to the "disable video" option included in many DVD-Audio players allowing the player's video circuitry to be shut down during playback to minimize signal traffic (and possible EMI) in the chassis.
At first glance, this sounds a bit odd, but on closer reflection it reminds me of one aspect of the 950's analog bass management design. In that case, the design team made a very deliberate decision to configure the crossovers in a specific way - a way that some people have disagreed with - in hopes of optimizing performance for most users. The crossover design came to light during the 950's initial roll-out, and even though there was the opportunity to change the 950 at that time they elected to stick with their original design because they felt it was the best solution for most prospective 950 owners. The bug we're looking at here could be similar - something they did intentionally to optimize stereo playback performance. Just a thought...