What's the difference between two asterisks instead of one asterisk when we refer to paths?

Earlier I was debugging my Spring 3 project. I was trying to add a .swf using

<spring:url var="flashy" value="/resources/images/flash.swf"/>

With my web.xml's ResourceServlet looking like

<servlet-name>Resource Servlet </servlet-name>
<url-pattern>/resources/*</url-pattern>

But unfortunately I was getting this error:

WARN org.springframework.js.resources.ResourceServlet - An attempt to access a protected resource at /images/flash.swf was disallowed.

I found it really strange since all my images in the images folder were accessed but how come my .swf was "protected"?

Afterwards, I decided to change the /resources/* to /resources/** and it finally worked. My question is... why?