Hi Paul, Ive noticed that too. I have a couple that also have that line across the top where there is no yellow color.
Ive even had some that looked more "aged" with a much darker color overall. It kind of looks like the paper had a layer of yellow coating over the lighter natural paper color.
Here are 2 Moreau's
(scanned with same settings, same time)
the one on left has the same line across top
(card on left is not mine anymore, thanks Jim
)
(scanned same time)