Felnar, you are right about natural armor bonuses. A Colossal dragon has a massive (-8) size penalty to AC because it is easy to have an attack strike the body of the dragon, and as a larger target it's much more vulnerable to touch attacks. However, as pointed out by Sundragon2012, many "missed" arrows fired at a dragon simply bounce off the dragon's scales, or–even more fun–get stuck an inch or two inside a scale, making the dragon a veritable pincushion without causing any actual damage. And any arrows which do go far enough in do little damage compared to the dragon's total hit points. Basically, if a touch attack roll would have succeeded, but a regular attack fails, the weapon strikes the target but fails to penetrate armor, natural armor, etc. So, while it's easy to hit the dragon (size penalty), those scales are so hard even high-level characters have a tough time getting an effective hit past them (overwhelmingly high natural armor bonus).