I’m reading it differently. 40% of ALL students don’t graduate in six years from the same school. But that doesn’t mean that 40% of the ones who STAY don’t; presumably that number would be higher.
The leaving out of students who transfer has always bothered me–I transferred and my D transferred. I graduated in four years and so did she–but we’re not counted. If everyone like us were counted, the six year graduation rate would be considerably higher than 60%.
(And then there are outliers like my S who graduated from an Ivy after a six year hiatus–so ten years after starting. The degree’s still good, 
).