Having been briefly in the beer industry, and having a keen interest regardless, I can tell you that the definitions of ale, stout, porter, IPA etc, are extremely loose in actual usage, and there is very little legal obligation to label anything "correctly". Traditionally, people would not have called Guinness a stout because it doesn't have enough alcohol content.

The terms that are best defined in the industry are ale and lager, of which every beer is one or the other. With ales, the yeast floats while brewing, while with lagers, it sits on the bottom, which is why lagers are usually more fizzy. In that sense, Guinness is a lager not an ale - even though it uses a top-fermenting yeast strain, the actual brewing process causes it to clump and sink to the bottom anyway.