I have the same thing going on with mine...Best I can offer is that it has to do with the outside ambient temperature. The warmer it is outside regardless of the temperature setting the air is going to be cooler. This morning, it was about 49 and my rear climate control was cranking out tons of warm air from the side set to 84 and the side set to 65 was cooler. My '12 X5D (with all of the things wrong with that thing) Functioned exactly the same way.

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2010 X5 3.0 X-drive Titanium Silver/Black
2018 Ford F-150 Platinum Crew
2010 528xi (His)
2017 M2 Manual (His)

Former Headaches:
2008 X5 RIP
2015 M235
2013 135is
2009 535xi Wagon (JUNK)
2007 530xi 6spd CPO Sport (Best Damn Car EVER)
2011 335xi M-sport Coupe 6spd (Sex on Wheels)
2010 550GT "CPO" (Junk)
2012 X5 35d "CPO" (More Junk)
2004 330i ZHP Sedan
2002 325xi Wagon
2000 323i Sedan
1998 M3 Sedan
1992 325i Sedan