Looking at the renderings again, it could be the home and away sides that are reinforced concrete and the end zones are steel supported bleachers which could then be removed and replaced when capacity is increased. The added capacity would be end zone seating and the seating similar to what's above the tunnels on each side of the end zones at RayJay. If the two end zones were originally concrete, it would be much more expensive to replace / upgrade when they increase capacity. Just a guess, like everyone else on here....