Initially the index is empty and only has the root block. As entries are added to the index, they get added to the root block until it becomes full.
Eventually, the root block has to be split and another level added to the index.
When that happens, the root block will get two entries that point to two leaf level blocks below it. Each leaf level block will then have some of the index entries.
As more index entries are added, one of the leaf level blocks will become full and will be split into two leaf level blocks. A new entry will be added to the root block to point to the new leaf level block.
As more entries are added to the index, more leaf level block splits will occur and new entries for them will be added to the root block. Eventually the root block will become full again and it will be split, adding a first interior level into the index.
Now there are three levels: the root block, two interior level blocks, and the leaf level blocks. The root block will once again have only a few entries which point to the two intermediate level blocks.
As more index entries are added, more block splits will occur and eventually the root block will become full again at which point another interior level will be added and the root block will end up with only a few entries.
So as entries are added, the root block goes from almost empty to full and then from full to almost empty.