I experienced this on MY07 but it seems to happen in the new cars too. Try this:

1. set both temp dials to 26c
2. set the air flow to 'face level'
3. dial down the passenger temp to 23c

In both my cars doing this changes the temperature of the air coming out of the right most DRIVERS side vent i.e. the passenger dial is controlling the right-most vent temperature (RHD car).

In the MY07 Stratsh1ts never bothered looking into it properly.

Any ideas? Is it a case of "they all do that sir"

N.B.
this is NOT the design limitation of +/- 4c, it's more fundamental than that. ============
2010 TDV8 HSE - ooo laa laaaaaaaaa
2007 TDV8 HSE - superb car and very reliable
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2012 - dearly departed, sadly missed.. to return in late 2013 with a new TDV8!