This seems to be an issue which often confuses customers (and core devs as well, sometimes ).
Basically defaultValue and/or selected options are only applied for new items, not for existing ones.

Quoting Mika:

"Default values are only applied when the item is new.

What would happen if we applied default values for existing items?
Authors could open the dialog, see the default value, and decide to simply close/cancel the dialog without realising that value is not saved.

It does not matter if the dialog definition changed.

Generally, when modifying dialog definitions, it might be a good idea to combine that with a new delta task, in the version handler; or at least with a groovy script (where then you can apply a default value to pre-existing content)."