The difference between UINavigationBar and UIToolbar rendering process

This is a basic question, but I realized that both of these bars have different rendering process. Why is that?

If I use an image specifically designed for UIToolbar items, and try to use it on UINavigationBar items, it doesn't render in the same way. It seems to be not rendered at all in the UINavigationBar.

So, I have 开发者_高级运维to create a different image for both of these types of bar. Is this a bug or an intended design? Am I missing something here? Is there any special setup to make both bars render their items in the same way?