[arch-general] downgrades when moving out of [testing]

I typically upgrade with -uu, and use [*testing]. I've noticed lately that there is a window when a package moves out of [testing] that it will be downgraded. For example, this happened when openssh-6.2 recently moved. Are the packages being removed from [testing] before being added to e.g. [core]? Does mirroring happen separately, but faster for [testing]? (I use kernel.org). Best, Matt