Your component.xml still contains this line:
<wb-resource deploy-path="/" source-path="/WebContent"/>
So if the path to your dev folder is /WebContent/WEB-INF/dev, it is still under WebContent and will still be published. If your component.xml previously said something like
<wb-resource deploy-path="/" source-path="/WebContent"/>
<wb-resource deploy-path="/WEB-INF/dev" source-path="/WebContent/WEB-INF/dev"/>
Then it was being redundant. Removing the second line doesn't stop the dev folder from being added since the first line still is a parent folder of the dev folder.
Hope that helps explain it.