My daughter's math teacher did that this past year, and my understanding is he has always done it. He does it because he is teaching 8th grade math to 7th graders, but they still have to know the 7th grade math because that is what the standardized test is about. He offers the extra classes to them as insurance, I guess. They all obviously did well enough with 6th grade math to just skip the 7th grade class.
I missed seventh grade math, myself, never learned how to do square roots.