I guess it has as much to do with thoughtless racial stereotyping as anything.
Many north-europeans have cranial dimensions that are (more-or-less) equal in yaw, roll and pitch directions (x-threading madly!!)so can be seen as "square" - albeit with rounded corners (and, pedantically-speaking, it would really be a "cube")

but that's how racial stereotyping goes - a rough approximation to a small percentage of the actual reality.