I think the idea is more, that there is no distinct level at which you can draw a line and can reasonably claim, that everyone beyond that line is or does x. That means, that you probably should not draw any conclusions from that line, even if there are differences in many cases on each side of the line. From each side if the line a person can surprise you by not being or doing what you would expect based on the line you drew, so you need to keep an open mind about the arbitrary line you drew.