Why has PCA getMean() 4 Dimensions?!

Hey guys,

I am using the pcl::PCA<pcl::PointXYZRGB> object to get the princinple components of a cloud. I was wondered to see that the getMean() method returns a 4-Dimensional vector. I would have expected either a 3-Dimensional or 6-Dimensional one, can anyone explain to me the reasoning of this? What should the forth dimension represent, in my case its always 1.

Best regards,
Zan