I don't know what the timeline is, honestly. I would assume that once Chaos got their foothold, "exports" out of Cadia would slow down or stop. But before that, there would be many regiments sent away who stayed away. Based on what I read, a regiment that survives their current campaign will just keep moving to where they're needed. Maybe all those Cadian regiments currently off Cadia were sent before the 13th Black Crusade?
Or someone lost the paperwork. Either one makes sense.