O-N-O bond angles in oxides of nitrogen

[![enter image description here](https://i.stack.imgur.com/vIpCH.jpg)](https://i.stack.imgur.com/vIpCH.jpg)


If we look at the bond angles of O-N-O in figure (c), (D) and (F), there not too much change in bond angle. Even though they are notable.


So my question is,


Why do we observe **very** small change in angles?


