I think you are over rating how NBA draft picks perform by draft position.
The #19th NBA draft pick is usually not a standout player. Here is the list of #19 picks from 1998 to 2008: Pat Garrity (10 years, 7.3 ppg), Quincy Lewis (4 years, 2.6 ppg), Jamaal Magliore (12 years, 7.2 ppg), Zach Randolph (17 years, 16.7 ppg), Ryan Humphrey (3 years, 2.3 ppg), Sasha Pavlovic (10 years, 4.9 ppg), Dorell Wright (11 years, 8.4 ppg) , Hakim Warrick (8 years, 9.4 ppg) Quincy Douby (3 years, 10.7 ppg), Javaris Crittenton (2 years, 5.3 ppg), and JJ Hickson (8 years, 9.5 ppg). Dorell Wright performed in-line with other #19 picks. By saying Dorell Wright performed below expectations, you are saying all of the #19 picks listed, except Randolph, performed below expectations.
Look at the #18 picks: Mirsad Turkcan (1 year, 1.9 ppg), James Posey (12 years, 8.6 ppg), Quentin Richardson (13 years, 10.3 ppg), Jason Collins (13 years, 3.6 ppg), Curtis Borchardt (2 years, 3.1 ppg), David West (15 years, 14.0 ppg), JR Smith (14 years, 12.8 ppg), Gerald Green (10 years, 9.6 ppg), Oleksiy Pecherov (3 years, 3.9 ppg), Marco Belinelli (11 years, 9.7 ppg), JaVale McGee (1o years, 7.7 ppg). Green has performed in line with the other 18th picks.