Charlie,
Most people stir their molten lead at times to keep the contents fairly consistent. This would eliminate the problem that Stump Killer mentions for future castings sessions.
While SK may have hit the culprit right, there are a few others that you might check as well.
I have a test for you to run. Do you have a micrometer?
If so, check the heavy ball against the others. It could be just a tad fatter away from the sprue line due to the failure of the mould to fully close.
Another possibility is that it was cast pretty hot and the sprue smeared rather than cut. This leaves a bit of extra weight. Check this area as well for any indication of this.
Good luck with your casting.
YMHS,
CrackStock