Each field of keywords.txt is separated by a single true tab. When you use multiple tabs it causes the field to be interpreted as empty. On Arduino IDE 1.6.5 and newer an empty KEYWORD_TOKENTYPE causes the default editor.function.style coloration to be used (as with KEYWORD2, KEYWORD3, LITERAL2). On Arduino IDE 1.6.4 and older it causes the keyword to not be recognized for any special coloration.
Reference:
https://github.com/arduino/Arduino/wiki/Arduino-IDE-1.5:-Library-specification#keywords
