Terrain heightmap non linear mapping issue

In my setup I like to import a RAW heightmap created in Photoshop. I've set the height of the terrain to 255. The general idea is that every value of grey maps to 1 unit of height. This would set for easy absolute control.

Yet it seems that grey values do not map linearly to the heights. Both 0 and 255 map to the min and max height of the terrain, so this works. Yet any value in between seems to be mapped slightly curved. I've attached a screenshot of said terrain with a linear gradient as heightmap.

Is this behavior intended and, if so, where can I read more about it?