Del Potro has 15 points from Viña, but replace it with his 19th result and you'll have 9 points less. Then replace Costa's points with his 20th result and you get 5 points less (anyways he'll have his Memphis points by then).
You are right about Memphis, I didn't know the ATP had increased the amount of points this year (last year, a player received 5 points for a 1st round and 15 for a 2nd).
I never said he was losing Costa's points in the 7th week, I just omitted Buenos Aires.
He loses 9 from Viña and defends the 10 points from Costa do Sauipe in Memphis.
He is not defending Buenos Aires points, but its irrelevant to his immediate ranking, because it was a non-countable tournament.