I’m not sure the two you mention are all that different, in practice.
But, when I used the term, I was simply referring to the fact that teachers will tend to teach to the 67% of the kids that make up the bulk of the bell shaped curve. If the bulk of the kids are operating at below grade level, the *typical* teacher will avoid having blank stares and discipline problems by teaching to that level.
Of course, GREAT teachers accept the challenge, go to war, and work their butts off to bring those kids up to level. :-)