There are so many other variables to contend with I don't think a few 10ths of a grain is going to make that much difference.
ironwood is correct, a few 10th at bad. a full grain in .50 is questionable. A void, is simply and air bubble that could have been trapped some where in the casting. If the bubble where to be in the exact center it wouldn't matter,, when I weigh ball, I find a group center of random weight. With a +/- 1/2 of one grain as tolerance.Then I put heavies with heavies, light with light,,and toss all that are more than 1 grain out of nominal back into the pot,,or just use them for scrap shooting or fouling shot's. Some say it doesn't matter. Guess what? it does, you have just removed one more variable. here's a trick in compition; shoot your out of tolerance ball during warm up and practice, your freinds will see your having a bad day. When the money lands on the barrel-head,,that's when you shoot the ball that weigh the same,;)
Just remember, the winner buy's the soda,,,,,,,,,,;)