I'd guess the sail orientation is not changing relative to the sun, so at perigee this provides an acceleration (because it's mostly in the velocity direction), which raises apogee. At apogee it provides deceleration (because it's in the anti-velocity direction), which lowers perigee. There are also probably other effects depending on the sail orientation that muck with the inclination and other elements. It's a little different than drag which is always anti-velocity.

I have limited KSP experience and an aerospace degree, so I'd defer to someone with more KSP experience.