This is what Chris Randall, the Eos developer just wrote in at GearSlutz:
"The background orb is an OpenGL shader; we're pushing the envelope a little on this UI. We're using all the tools available to determine the proper screen scaling, but due to the vagaries of OpenGL, a very few users are experiencing this mildly weird two-orb issue. (You're actually the second person I'm aware of that has had it.) It seems to happen on Retina Macs that are driving a non-retina monitor, so the OS is informing the plugin that it's a Retina system, but the graphics card is rendering it non-Retina. In any event, this is just a visual thing and has nothing to do with the operation of the plugin. That shader has zero to do with the sound of things; just eye candy."
I guess my MacBook problem is quite similar. MacBooks are not made for music purposes.
Meanwhile, my MacBook Pro (quad i7) has no prob at all, displaying that nice swirly blob :)