Under Community Actions > Edit Community > Forums you have the following settings:
Communities overview page > Show topic list by default
Forum application default view > Show forum list by default
From the Overview page if you click on the "View All" link in the Forums widget the application displays the list of the topics, not the list of forums as specified in the "Forum application default view" setting. It seems that since the community forums are being accessed via the Overview page the "Community overview page" setting takes precedent over the "Forum application default view" setting. When you click on the on the Forum tab in the community, you're taken to a list of the community forums as expected.
This seems to be working as designed, but it may be a little misleading to end users. In the future, we would like the "Forum application default view" to take precedent over the "Communities overview page" setting even when accessing a community's forums by clicking on "View All" in the Forums widget on the Overview page.