问题如下：
In terms of default probabilities, how consistent have credit ratings been between different geographies? How would expectations for a European BBB-rated firm differ from those of a BBB-rated firm in the United States or in the emerging markets?
选项：
解释：
European companies (particularly those with investment grade ratings) have performed better than U.S. companies. Emerging Markets companies with AA and A ratings have performed relatively well.
请问下老师，这道题背后的原理是啥？为啥欧洲和美国的会存在这个差异？另外，再请教下，新兴市场和成熟发达市场有啥区别？谢谢
