Maybe they think we are all so used to violence in life and on tv that it is ok at 15's. There is no swearing or sixual content also.
There's loads of swearing in Rage. The Wasted clan use a lot of colourful language - heard the F-word a few times in different contexts, and some more British swear words such as Boll***s and Wan***.
I'd also say the game is violent (blades stuck in heads, decapitations, bodies exploding to mush... and the way the enemies react to being shot is pretty realistic which makes it more brutal.
Also surprised it's a 15.