what exactly is string length?

[mw via Digitalmars-d-learn]

https://run.dlang.io/is/B4jcno
---
import std;
import std.conv : text;


void main()
{
    char[6] s;
    s = "abc";
    writeln(s, s.length);  // abc6, ok it's the static array's 
length

    string t = text("head-", s, "-tail");
    writeln(t, t.length);  // head-abc-tail16, why?
}
---

Why the last output is 16 instead of 13, t's type is string here.