One would have to assume they did not sign these things at the same time. Would be to messy. So it must be the glossy material. Frampton's CD was manufactured in Mexico and inserts run the quality gamut. Probably no standards for the insert material means various good and bad results. Hey, at least they signed in a good spot where you can see it.
And it affected the resale /flipped price you can pick these lighter signed cds up for £39.99 on e bay here in the UK whereas the darker ones are in the £70 to £90 range