Why is there a 14th Noah? This question could contain spoilers.

In Chapter 187, it was stated that after the Millennium Earl was killed, the remaining twelve Noahs repopulated the world - That means that there were only 13 Noahs. However, Nea Walker is referred to as "The 14th Noah". Where did he came from?
Unanswered in the manga as of now, but it is entirely possible that there are 14 Noah and the Earl only mentions 13 (including himself) because the 14th apparently dislikes the Earl, as he wants to dethrone him and take his place. Chapter 218 hints that a "Mana D. Campbell" may be responsible for the existence of the 14th, although i can´t really call the Earl in his current state of mind a reliable source.