Your polar call is only plotting half of the evaluated transfer fuctiion.
Try this:
num = [10];
den = [1 -2 1];
G = tf(num,den)
% %% code number one
[MAG, PHASE] = bode(G);
PHASE = PHASE(1,:);
MAG = MAG(1,:);
figure
polar(PHASE*pi/180, MAG)
hold on
polar(-PHASE*pi/180, MAG)
hold off
% %% code number two
figure
nyquist(G)
The nuquist result is more appropriate than the polar result, however the plots are the same if you plot both parts, and include axis equal with the nyquist plot.
