bermuda triangle

So today we were learning about acute renal failure. During PBL (weekly tute groups) we establish a list of learning issues which we go away, form answers for and then return the following PBL to educate each other with. Talking about the different types of renal failure fell on my fellow PBL Q-er, Joe today.
JOE: So Acute Renal Failure can be classified into 3 different causes: Prerenal, Intrinsic or Post-renal.
*He draws a little flow chart*
JOE: Pre-renal causes consist of...
*he adds to the flow chart*
JOE: Intrinsic consists of...
*adds more*
JOE: And finally post renal causes are...
*adds*
JOE: Essentially, I read that Pre-renal and Intrinsic consist of 75% of all cases of Acute Renal Failure, which you would assume meant that post-renal makes up the other 25%... It doesn't.
Shock horror that it turns out that medical researchers can't add.
JOE: Post renal failure does in fact account for only 5% of cases... which leaves some kind of bermuda triangle.
*Draws a triangle outline as an offshoot from the flow chart*
ME (thinking): Is this going to be on the exam?