weird behave of Array?

[dmm via Digitalmars-d-learn]

import std.stdio : writefln;

void test(string str)
{
  writefln("%d, %d", str.length, str.capacity);
  str.length = 10;
  writefln("%d, %d", str.length, str.capacity);
}

void main()
{
  string str;
  str.reserve(1024);
  //str.length = str.capacity;
  writefln("%d, %d", str.length, str.capacity);
  test(str);
  writefln("%d, %d", str.length, str.capacity);
}

The output is:

0, 1358
0, 1358
10, 1358
0, 0

If I uncomment the line str.length = str.capacity;

then the output is:

1358, 1358
1358, 1358
10, 0
1358, 1358

why?