Graduation rates (light on the graduations)

A high school with a four-year graduation rate below 60 percent is easily labeled a dropout factory, because we find these results clearly unacceptable.

I look at this graph and I struggle to decide what six-year graduation rate is unacceptable, and I struggle to explain why the standard should be lower for universities than the one we apply at the secondary level.

Please, do add your thoughts on this conundrum in the comments section.