Is the inner fold 'iz' supposed to be the same as the 'az'?

When using `foldmethod=marker' 

```
 " {{{
 I was expecting that viz will select one this line
 " }}} but it also selects the start and the end of the fold
```

It's there any plan to change the iz to select just the inside of the fold instead of the complete fold? Or is it done on purpose ?
