Why is the bond angle of N-N-N in hydrogen azide not 180 degrees?

Chen and Wang (2009) have reported an $\ce {N-N-N}$ bond angle in the hydrazoic acid molecule of $171.5$ degrees using B3LYP/6-311G\*\* model. The wikipedia page for the molecule also reports a similar bond angle of $171$ degrees. However, this seems to go against predictions by VSEPR theory for the case of two regions of electron density (which gives a bond angle prediction of $180$ degrees). It is known that VSEPR theory is limited in its theoretical predictions. However, I would like to understand what accounts for the discrepancy in the two. 


**Reference**


Chen, F., & Wang, F. (2009). Electronic Structure of the Azide Group in 3′-Azido-3′- deoxythymidine (AZT) Compared to Small Azide Compounds. Molecules, 14, 2656-2668. doi:10.3390/molecules14072656


