Do not think for a second that a sold slug will behave exactly the same as shot during the peak pressure of the firing sequence. 12 bore in itself is a relatively low pressure cartridge and slight variations in load formulas can give vastly different pressures, when all other things are equal. Even a change in wad can cause dramatic effect. Exactly why shotgun loading manuals are massive, relatively!

I'm presuming the ingots you have are recently cast? Are they pure lead? The slug appears lop sided, butbthat could be the effect of the camera.