No kidding? I've never seen that.

As it was told to me, impurities in the lead do not allow it to shrink (when cooling) as much as lead does. So impure bullets come out oversized.

Perhaps this varies among the kinds of impurities one is likely to get in their lead?

If you used commercially obtained lead, my guess is that any impurities that were in it were intentional, and probably help fill the mold. I could be wrong on that.