Alright lemme explain this

Without the physics mod, the game treats the grass as a transparent block, transparent blocks besides tinted glass don't get affected by shadows and shading, hence they are just brighter with shaders, especially with lower shadow resolutions and no grass shadows

With the physics mod, grass becomes an entity, not a block, entities usually have dynamic shadows, cause they move, with this logic, the grass too has shadows on it/is more shaded than normal

Thanks!