Well, internationally, junior is an age level, not a skill level, so they aren't really skating down. In general, they aren't as good as the seniors- because if you are good enough to skate as a senior, you probably would. But in countries like the US or Japan, where there are lots of good senior ladies (for example) some of the younger "senior-ability level" skaters will skate as "junior-age level" skaters.
The problem is that the same word is used to mean two different things.
(Denis Ten, who skates for Kazakstan, I think- went to Junior Worlds and Senior Worlds last year. He is a junior by age, but his ability allowed him to be a senior, by skill. There aren't a lot of skaters in his country, so he just did both.)
An example of an American skater would be Richard Dornbush- he skates as a senior in the US (and just got silver at nationals), but all year he has been competiting as an international junior- on the JGP. Now, however, because he medalled, he'll be going to regular Worlds, not junior worlds. However, the US had so many men on the Senior Grand Prix, as an unknown, he wouldn't have been able to get assignments, so he skated as a junior. It's likely if HE had been in third and not second, the US may have sent him to junior worlds, and leap frogged Jeremy Abott over him to go to senior worlds. But Ross Miner, who placed 3rd, is too old to be eligible for junior worlds- so the "leap frogging" doesn't really work.