text: Because it's a generality. If it's a generality we'd expect to not see it be unique to Islam, nor not to affect Christianity. Rather, we'd expect to see many Muslims be better than their religion commands, and many Christians be worse than their religion commands. However, in general, the fact that one group is acting in accordance with their religion, and the other isn't, would cause us to see a general distinction. And what do you know, apostasy isn't a serious issue in the major Christian areas, while it is in the Islamic areas.