This is actually an interesting observation. I would respond that vowel sounds are more similar than consonant sounds. They all require similar mouth shapes and are all drawn-out, guttural noises. Consonants, on the other hand, force the mouth into all sorts of different shapes. B, K, SH, and W are all totally different sounds that seem harder in theory to mix up. Granted, in this case, N and M are some of the more similar consonants and seem more likely to get confused with each other.