Atan function

[Laurent Berger]

In this program

clear;
N=256;
n=0:N-1;
Xr=zeros(1,N);
f0=2;
Xr(f0+1)=1/(2*%i);
Xr(N-(f0+1)+2)=-1/(2*%i);
Xr=Xr.*exp(%i*%pi*2*6*n/N);
xr=ifft(Xr);
disp (atan(imag(Xr(11:12)),real(Xr(11:12))))
disp (Xr(11)-Xr(12))

results are
  0.    3.1415927

     0


I don't understand why two numbers with a difference of 0 can have a 
numerical phase of 0 or pi .
In matlab results are
0 0
0


!Scilab Version:             5.3.1.1299160056 
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!Operating System:           Windows XP 5.1 
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!Java version :              1.6.0_18 
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!Java runtime information :  Java(TM) SE Runtime Environment (build 
1.6.0_18-b07)     !
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!Java vm information :       Java HotSpot(TM) Client VM (build 16.0-b13, 
mixed mode)  !
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!Vendor specification:       Sun Microsystems Inc. 
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