Some common traits of Africans are larger lips, pale skin on soles of hands and feet.
Common traits of Chinese and Japanese are light copper skin, black hair and narrow eyes.
Irish typically have red hair.
Scottish are known to be on the heavier side, typically blonde/brown hair.
It's entirely feasible that this pau'an has a wider head.
As for why he looks this way on camera -
In episode III a pau'an was seen talking to Obi where he seemed to have a narrow head. The actor was on stilts/raised platform to make him seem taller, the garment they gave him made his head appear more narrow than he actually was.
Doing this for a sith who is expected to be doing intense lightsaber combat wouldn't work.