User blog comment:Arbington/The Next Cycle/@comment-5267262-20120803203730/@comment-1221773-20120803225451

From the image we've got, it looks like a Destroyer's upper carapace has been removed and/or opened up and saddled with the cannon, leaving some interior mechanisms exposed. From the Rannoch battle, we know that the Destroyer's armor keeps it well protected until it opens it up to fire its laser, making it vulnerable to the orbital strikes.

But yeah, on the whole the portrayal of Destroyers is inconsistent.