Sorry...I'm actually having difficulty seeing the screenshot. I guess, as you say, it's all different apps, but they appear to be the same size?
When I've seen this, I've seen it with multiple windows of the same app open, because by default, they're the same size. But my understanding is that it can also happen if other apps are all exactly the same size?
I believe, but I'm not sure, that if the apps are very slightly different in size (i.e. by a few pixels), then you get the "X" configuration and not the horizontal row.... The "X" config shows the windows as much larger without any obvious disadvantage, and Expose uses it unless you end up in this particular condition... I guess I don't know that it's a bug, but it seems to me like preserving the "X" would be better?