From the I=1 state, we got three different magnetic hyperfine levels, with state ml = +1 the lowest > ml = 0 > ml = -1. From given picture, we also see that each quantumstate ml has a different orientation. For a low temperature, the lowest energy levels are most occupied (so for ml = +1) which corresponds to an orientation, dominated along the positive z-axis. This results in a polarized symmetry along the positive z-axis.