Ok, this is something I probably should have been able to figure out without the help of the internets, but today, PBR said “niner,” and I started wondering why pilot types say nine that way. “Niner” is the pronunciation of nine in the unambiguous NATO phonetic alphabet (Alpha, Bravo, Charlie, etc.). Other numbers with designations other than their english pronunciations are three (“tree”) and five (“fife”).

The Wikipedia page has many more interesting facts about the NATO phonetic alphabet.