I believe the way it's supposed to be justified is that you can take a 10 on anything. This represents the "What's a bear? Oh yeah." The 15 would include the +5 bonus you get from being trained in Knowledge: Nature, I guess. So then the taking ten would be like "Oh yeah Bears has Claws."
Of course, this doesn't explain why you only know how bears attacks if you're trained in Knowledge: Nature. So, in D&D now, you need a formal class to learn that bears do not carry swords, bows, or firearms. Awesome.