One morning in April 1992

[Inquisitive Raven]

I can see a couple of possibilities, neither of which really apply to Snoopy. One being that it affects the cat's ears, and the other being that the fur right above the cat's eyes is much thinner than elsewhere, allowing you to see through to the underlying skin, an effect that I think is much more noticeable in solid colored cats like Luna and Artemis.