The Hoplon Shield has 5 evasion and the "heavy" class Tower Shield has 10. Shouldn't lighter shields have higher evasion than heavier shields since it'd be easier to move around with them?
No. Evasion is also a catch-all for things like staying at range with spears and bows, and deflecting attacks with your shield. You're taking this too literally.
Imagine evasion not as dodging, but as in being able to avoid an attack by the means which the item you use enables you to, such as deflection (shield), distance (bows/spears), and then mobility/dodging (light armor, light weapons and such)
The Hoplon Shield has 5 evasion and the "heavy" class Tower Shield has 10. Shouldn't lighter shields have higher evasion than heavier shields since it'd be easier to move around with them?
Not to mention that a hoplon shield is meant to cover from the shoulder to the knee, as well as cover the person next to their left. Granted, the Champion isn't fighting in a phalanx, but they are using a large wooden shield that's covered in bronze and meant to be used in a group. Its meant to take hits and deflect it away from the Champion.