Basically, a table is a "heap".  We stick data WHERE EVER. an index is a structure, we have to stick data where it belongs.

When we take an index block off of the free list -- it is "empty".  We will then totally fill it up  until we have to split it.  It only goes back onto the free list when it is empty again.  We cannot  put a block on the freelist unless it is empty -- the fact that the INDEX IS A STRUCTURE, prevents us from sticking just any data on the block.  Only data that can go onto that block can go there.  
It is just "not a heap". 

The first important advance in Oracle optimization technology follows from a simple mathematical observation:

You can't extrapolate detail from an aggregate.

Here's a puzzle to demonstrate my point. Imagine that I told you that a collection of 1,000 rocks contains 999 grey rocks and one special rock that's been painted bright red. The collection weighs 1,000 pounds. Now, answer the following question: "How much does the red rock weigh?" If your answer is, "I know that the red rock weighs one pound," then, whether you realize it or not, you've told a lie. You don't know that the red rock weighs one pound. With the information you've been given, you can't know. If your answer is, "I assume that the red rock weighs one pound," then you're too generous in what you're willing to assume. Such an assumption puts you at risk of forming

conclusions that are incorrect—perhaps even stunningly incorrect.

The correct answer is that the red rock can weigh virtually any amount between zero and 1,000 pounds. The only thing limiting the low end of the weight is the definition of how many atoms must be present in order for a thing to be called a rock. Once we define how small a rock can be, then we've defined the high end of our answer. It is 1,000 pounds minus the weight of 999 of the smallest possible rocks. The red rock can weigh virtually anything between zero and a thousand pounds. Answering with any more precision is wrong unless you happen to be very lucky. But being very lucky at games like this is a skill that can be neither learned nor taught, nor repeated with acceptable

reliability.