No, no, no!
You got it all wrong, Blue Toad. Yoshiman222 is correct all along. There are only 14 "bosses" and seven "stages," with two bosses per stage. How else can there be 14 bosses with only six boards? DK and Diddy Kong are bosses, but not in the way you would fight them, but in the way they would assist you. They're bosses in a completely different manner. 12 bosses are there to harm you, while the last two, DK and Diddy Kong, are there to help you. Why would DK and Diddy Kong would want to betray Mario and his friends?