On Coworkers

I think that at every job, there is some sort of law of nature that requires at least one coworker to be a little bit off of the normal average bell curve. I worked last night at my job as a pseudo-waitress, and I have a pretty good relationship with most of the people I work with, at least to the point that none of us look down on each other as being the rookie, and we can laugh at the ridiculous things that people do. But no matter what mix of people I work with on any given night, there is always one person who doesn't quite pull their own weight, as it were. I don't just mean someone who is new and doesn't know what is going on, I mean someone who just doesn't quite understand the doing of things, even after they are explained to them. Is this normal? Has anyone else experienced this phenomenon?