Why does this work? (SetLength)

Hello,

I'm playing around with the SetLength function und I wonder why the 
following little test app works:

{01} program setlengthtest;
{02} var strarray:array of string;
{03}    i:word;
{04}
{05} begin
{06}    setlength(strarray,2);
{07}    strarray[0]:='1. String';
{08}    strarray[1]:='2. String';
{09}    strarray[2]:='3. String';
{10}    strarray[3]:='4. String';
{11}    strarray[4]:='5. String';
{12}    for i:=0 to 4 do writeln(strarray[i]);
{13} end.

To my understanding there should be an error in line 9, because the 
array index is exceeded.
If I compile this little app on MacOS X 10.4.8 with FPC 2.1.1 it seems 
to work just fine. So how to set a strict boundary of a dynamic array?

Any ideas?

Thanks in advance,

Dennis Schramm