How does setting up the Expense Category work?

I have my Expense Categories set up with the Expense Account defined on them:

But when I enter an Expense Line with this Category, the Expense Account is not completed?

Surely the purpose of defining the Account on the Expense Category is to ensure the correct account is added to the Expense Report Line?

One thing I did notice is that when I search for the account in the drop down, they accounts are shown without the account number:

Whereas the setting on the Category (shown above) includes the number? Is this a potential problem?

Is there something I'm missing? Or do I need to build a workflow / client script to enter the correct account???