That's because you can choose the actual default via right clicking on an empty space on the button bar. It allows TC to dynamically change icons when you switch icon libraries.
Please read my reply. DefaultLib stores what you have chosen as default icon library.
The button bar dialog will show wcmicons.dll, but internally TC will substitute it for what is stored in DefaultLib. This way you don't need to change all the buttons when you want to try a different icon library.