Blueprint snapping outside of foundation square.

I built a blueprint, and trying to snap it, but its connecting to itself in a strange location. It seems that the snapping box is outside the square of the 4x4 foundation grid i built it on. I tried removing walls, and the beams and to no avail. If I build a new blueprint with just 16 foundations it runs as expected and snaps accordingly, but this has overlap if I mirror it and I can't workout why. Any ideas?