Your chocobo will get 12 SP for reaching rank 18, 13 SP for rank 19 and 14 SP for rank 20 - I guess that you expected him to just continue getting 10 SP per rank (and didnt notice you got 11 SP for rank 17 already), so thats where your missing SP will most likely come from.

I can assure you that a chocobo at level 20 will have every skill unlocked.