How many Tabs do I get after striking Tab key under VIM Insert mode?

 How many Tabs do I get after striking Tab key under VIM Insert mode?

I am learning C programming.

I met a problem about vim configuration.

The following is my vim setting written in .vimrc

set nu
set shiftwidth=4
set tabstop=4
set softtabstop=8
set autoindent
set cindent
set smartindent
syntax on

In order to see where the tabs are, I have used :set list

#include <stdio.h>$
int main(void)$
{$
^Iprintf("ab\n");$
^Ireturn 0;$ 
}$

Now in line 4 under Insert mode, I want to see how many tabs I can get after I strike Tab key in diferent locations.

Here's the result:

When I strike Tab key after n, I get ^Iprin^I^Itf("ab\n");$

When I strike Tab key after \n, I get ^Iprintf("ab\n^I^I");$

The above two situations are quite understandable to me.

But when I strike Tab key after a, I get ^Iprintf("a^Ib\n");$

This is beyond my understanding.

Can anybody explain why there is only one tab replaced?

I thought there should be two because softtabstop takes up 8 spaces which is two times tabstop (4).