If I create a cube, move it away in the x direction, and zoom all the way in toward it (i.e. from either left or right), its center is still aligned with the origin:
Same with the z direction. But if I move the cube in the y direction and zoom in toward it (i.e. from either front or back), its center creeps below the xy-plane:
I am new to Blender so I'm not sure if I'm somehow misunderstanding the orthographic view, but I could not think of a possible reason it should be behaving differently on the y-axis than it does on the other two. When I tried the same thing in Rhino 3d, for example, this did not happen. I am just wondering if this is an indication of some more general feature of orthographic view that I need to be aware of and account for.