Actually Ed, I believe the chain plates are angled with the shrouds. When you are looking at the ship from this oblique angle, and not straight from the sides, the inward bend of the chain wales as they pass over the channel tricks your eye into thinking they are vertical. I have noticed this effect from gazing at my last model. The vertical angle when observed from a position in front of the channel, perpendicular to the hull, removes the distortion caused by the athwart ship angle, and there is a subtle but detectable angle in each chain plate which keeps it parallel with the shroud in that plane. Here is another clue. If the chain plates were indeed vertical, then the last segment of the chain plate assembly, the plate that is anchored to the hull by two bolts, would be in perfect vertical alignment with the edge of the gun ports. Even in the photo below, you can see this is not the case. Look at those plates on the extreme right, and the extreme left. They are not parallel to each other, or the lines of the adjacent gun ports. The sweeping change in the vertical angles of the chain plates are subtle, but they are there.
Just don't stare too long, or your brain may explode.
View attachment 288999
PS: Also take note that the spacing of the shrouds is not even, but that which allows the chain plates to be anchored between the gun ports. The gun ports determine where the shrouds can go.