[Dovecot] dspam plugin and updating of headers?

Forgive me if this is a stupid question. I know that when I drag a message to the SPAM folder, it is being trained properly, since the stats indicate so. Dragging back to the inbox retrains it as ham. I happened to notice that despite the fact that dspam says this in the debug log:

"appending header X-DSPAM-Reclassified: Spam"

nothing is actually being done to the message. Looking at the debug log for a newly delivered message, I can see it is calling dovecot's deliver agent. I am guessing here that this is just the way it works? e.g. the plugin detects a message being moved from inbox to inbox.spam and calls dspam with the requisite arguments to retrain the message, but the message isn't really being modified?