Perhaps because the first one (where she's sitting with Carbuncle) was done awhile back, around the time summoner/arcanist was just announced, and the "job wheel" pic is more recent, showcasing the new jobs AND the new class that's coming.

Notice how the color schemes between summoner and the elezen in question are similar? It wouldn't look as good, imo, if they just threw in the old elezen pic with the white robe into that pic, as the clothing she's in is pretty bland compared to the pizazz of the clothes she's wearing in the "job wheel" pic.

All promotional stuff..you know, artistic license and all.