well ona simple level, metal is a great heat conductor and carbon is not. Thus the metal bar can be colder due to a bit of a heat-sink quality, where the carbon bar will locally heat up from your hands, but shouldn't transfer like the metal does. However, having a solid cork or foam bar wrap would likely mitigate much of the issue either way.
I've got foam tape on my bike and have been riding around all winter with just a pair of fleece gloves. The wind had more effect on my hand comfort then the bars.