DC Untied going to Copa Sudamericana

DC Untied has accepted an invitation to South America's second biggest competition the Copa Sudamericana. The draw for games and dates will take place later today.

United won this honor after finishing third in the CONCACAF Champions' Cup this year. Great for them, but does anyone know how did they ended up getting the third spot over Houston? The Dynamo lost to Pachuca who went on to win the Champions' Cup so why did they not get the invite? Was third place decided by goal total/difference during the tournament?

Not trying to take anything away from DC, just trying to figure out why they got third.

Anyway, it is always great to see a MLS side play in meaningful international competition. Go United!