Just an example of the US Ladies.
Suppose Mirai and Rachael score 13 (6th & 7th) place at Worlds 2011. They total 13 and both will be qualified to skate in 2012. That 13 is not enough to send another skater to Worlds 2012.
I believe the USFS is stuck in a catch 22, or they must have a Winner (1st Place) to automatically have a 3rd skater whose placement will have to add up to 18 for 2013.
Yuna KIM can have 3 skaters but the Korean Fed has to send them to the Quali Round.