Fragrance concentration does not just mean the strength of a fragrance, but also the behaviour of it. When a fragrance is available in different concentrations, there is a difference in the composition which will not always be noticeable at first sniff. The composition allows the fragrance to behave differently, for example, Eaux de Toilette are bright, light and diffusive in comparison to Eaux de Parfum, which are warm, soft and 'thick'.
An ideal way to think of fragrance concentration is as a fabric. Three fabrics that are all black will not all share the same appearance, silk being shiny, velvet being soft, and cotton being flat or matte. The different texture changes the appearance and feel. So an EdP may not just be a stronger version of the EdT, but have a completely different feel.
Chanel are a house notorious for having several different versions of a fragrance, in fact, No5 was launched as a Parfum in 1921, with Eaux de Cologne and Eaux de Toilette in the 20s, a monstrously potent Eaux de Parfum in the 80s, then the recent addition of Eau Premiere launched in 2008. And many mainstream fragrances are launched in several concentrations to draw attention back to the brand - a few recent examples include YSL's Elle, Armani's Code, Estee Lauder's Sensuous and so on.
Parfum (or Extrait) is the form of fragrance with the highest concentration of 'fragrance oil', at between 15-30%. The Base notes are the focus of the composition, and thus it is much longer lasting, creating a fragrant trail but wearing very closely to the skin. Even though it is more concentrated, it doesn't necessarily smell louder or stronger.
Eau de Parfum (also known as Esprit du Parfum or Parfum de Toilette) is usually around 12-15% concentration and is intermediary between Parfum and Eau de Toilette. The focus is on the Heart notes, often accentuating the warmer notes in the fragrance. It is usually more radiant than a Parfum, and appears to have more of a life to it.
Eau de Toilette ranges anywhere between 4-12% concentration, and is focused on the Head notes. As such an Eau de Toilette is more diffusive and alive on the skin, and tends to be more tenacious than other concentrations.
Eau de Cologne is a low concentration, around 1-2%, and is a light way of applying fragrance. Usually applied as a splash throughout summer days to freshen up. It is also a specific family of fragrance with citrus, neroli, rosemary and other herbal essential oils.