I struggle to believe that most scammers in the world are that good that make deliberate mistakes in their letters to reduce false positive rate. More realistic solution is that the average scammer is not well educated, he makes mistakes unintentionally and out of pure luck it works well for him.
About why they say they are from Nigeria, I'd say that simpler the scam is, easier it is to pull. If you communicate via voice, it is hard to properly mask accents. If you are sending something via money transfer, destination address is obvious as well.