An article has a couple of definitions in MediaWiki terms, owing to different opinions about what an article is; some maintain that an article is a page in the article namespace, which is a fair assertion. However, MediaWiki uses a specific definition of an "article" when determining whether or not to include a page in the "content pages" statistic on Special:Statistics.
In MediaWiki terms, an article is a page in the main namespace, or a content namespace (see $wgContentNamespaces) that is not a redirect and contains at least one internal link. When there is an embedded image this also counts as an internal link.
The setting $wgUseCommaCount can be used to change the requirement of having a link and to have a comma.
See also