It makes sense in a homeschool situation. Or, in other words, the reverse makes no sense. There's no reason I would, say, teach my child fractions, have them be confused and doing them all wrong, and then say, "Welp, that was all the time we have for fractions! On to geometry. You get a fail on fractions." I wouldn't hold my child back a grade in homeschool in some punitive sense, and, conversely, there's no time pressure to move up at a certain pace. We can speed ahead of things they've got down pat, and slow down for the more frustrating bits.

But then I remember my experiences in school, where teaching to mastery was not the norm. You kept up — or you flunked out. I was strong academically, so my two personal examples were in art and physical education. Fortunately, both were graded more on effort than skill, but I remember being criticized in art class more than learning how to complete assignments. In P.E., I remember being ignored most of the time. I wasn't worth bringing up to scratch, I suppose.