Any thoughts?

[Tom Albrecht]

Scan nonlinearity is often the culprit for brightness variations, but when scan nonlinearity is the cause, usually you can see the nonlinearity of the scan directly, as well as the change in brightness that goes with it. When there is horizontal ringing, for example, you can usually see "wrinkles" in the picture along with the brightness variation. The wrinkles, of course, are only visible if there is content in the image that lends itself to recognizing it, like lettering, some kind of grid-like image, etc.

Looking at the second picture you posted, which has lots of nice lettering all across the screen, I don't think I can see any evidence that the lettering in the bright area is compressed horizontally compared to the darker area.

So it looks to me like scan nonlinearity isn't the cause in this case.