Often "laid off" is "fired" (not often as in > 50% of the time, but often as in not-rare). Companies will use "layoffs" as a way to get rid of the dead weight w/ less risk of being sued.
You are assuming the company knows who the dead weight is. I have seen more than a few times that someone is laid off, and only after he or she is gone do their managers figure out exactly what they did. Usually to their chagrin that there is no one else who can do it.
Oh what I mean is that people who would be fired tend to get held on to until the next layoff and then released that way.