why luaZ_fill discount char being returned

luaZ_fill discount the byte ,but luaZ_read put it back; it seems unuseful;

why code do not write like this:

int luaZ_fill (ZIO *z) {

 ....
 z->n = size - 1; /* discount char being returned */
 z->p = buff;
 return cast_uchar(*(z->p));
}

size_t luaZ_read (ZIO *z, void *b, size_t n) {
 while (n) {
  size_t m;
  if (z->n == 0) { /* no bytes in buffer? */
   if (luaZ_fill(z) == EOZ) /* try to read more */
    return n; /* no more input; return number of missing bytes */
   //else {
   // z->n++; /* luaZ_fill consumed first byte; put it back */
   // z->p--;
   //}
  }
  ....
 }
 return 0;
}

why need discount and put it back ?