Three boxes inexplicable force???

I put three 64x64 boxes, this box sprite has the physics behavior, down on a single layout, they are widely spread apart, such that there is no overlap between any of the boxes, and yet when I start the simulation, they immediately "repel" each other, as if there is some gravitational mass that I'm missing.

Changing density seems to have no effect. What the heck is going on?

thanks!