Changing private final fields via reflection

class WithPrivateFinalField {
    private final String s = "I’m totally safe";
    public String toString() {
        return "s = " + s;
    }
}
WithPrivateFinalField pf = new WithPrivateFinalField();
System.out.println(pf);
Field f = pf.getClass().getDeclaredField("s");
f.setAccessible(true);
System.out.println("f.get(pf): " + f.get(pf));
f.set(pf, "No, you’re not!");
System.out.println(pf);
System.out.println(f.get(pf));

Output:

s = I’m totally safe
f.get(pf): I’m totally safe
s = I’m totally safe
No, you’re not!

Why does it work by this way, can you please explain? The first print tells us that the private "s" field has not been changed, as I expect. But if we get the field via reflection, the second print shows, it is updated.