Sorry I didn't really get your question, I think. Del Potro can play one more tournament without dropping any points, the Paris Masters. It is not listed yet, but it will complete his 18. The profile assumes that the player will enter all TMS events they are eligible for, and holds an empty place for them. If you look at other players that will qualify like Nadal and Djokovic, they also only have 17 listed now, but will add Paris.
Now, everything is clear