I assumed -devel is for "release early, release often" for those comfortable with having to flash their N900s if, or more likely when, these go wrong. Once the developer is satisfied that his package is reasonably stable then it is moved to testing and then those who don't want bleading edge but are willing to have a little instability can find any major shop-stoppers before the rest of the world gets it.

Basically this is a variant of Debian, but leaving it up to the delevoper to promote to -testing and using voting to get from -testing, unlike Debian that uses severe bugs to handle migration from unstable to testing.