In sqlservercentral blog I've encountered this phrase
Symmetric keys are deterministic when created, meaning that the same parameters run in different databases will result in the same key. That means that the same key in a different database (or instance) can decrypt data that was encrypted in your production instance. Keep control of the parameters used to create symmetric keys and secure them. That means watch out for storage of these items in source control, in installation files, upgrade scripts, etc.
I can't understand what it tries to say. Symmetric keys are protected by DMKs, so how can they be the same on different instances?