In the section Settings > Mail > Templates and signatures, you are not setting the default email address for each template, but rather the default template for the specific account.
When you click New > Mail and then choose a template from the menu, eM Client always uses your default email account, regardless of which account you selected when creating the template.
However, if you are currently active in the specific account (or one of its folders) in the left panel and then you click only “New”, that template will be used.