The percentage of interns that get a graduate position. What are the reasons for not getting one?

STEP
#1

I understand that the Big 4 have high percentages in terms of offering their interns graduate positions. E&Y had 84% last year.

What are the reasons for not getting a graduate job offer? Anyone have any experience? Is it really bad feedback or not performing at a certain standard or just being a complete idiot during your placement?

A lot of firms have an additional step, after the internship, such as a final interview which is combined with any feedback. But at a firm like E&Y, an intern goes through the exact same process as a graduate without the guarantee of the graduate job, even if they are both successful.