If I understand you right,
your question refers to the DotNetNuke 3.1.1 version and not to the forum module.
If you have renamed the develelopment.config to web config, the object qualifier is set to dnn_ as default in the web.config.
And that should be the reason why you have dnn_ as an object qualifier in the table name.
In the file release.config is no object qualifier used as default.