Out of curiosity, last night I weighed and sorted a batch of .575 balls cast from a single cavity Lee mold, which is listed as 285 grains.

Here's how they broke out.

9 (3%) were under 283 grains (discarded those)
56 (20%) were 283 to 283.9 grains
84 (30%) were 284 to 284.9 grains
80 (29%) were 285 to 285.9 grains
46 (16%) were 286 to 286.9 grains
5 (2%) were 287 grains or more (discarded those)
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280 TOTAL

The balls were from two different casting sessions, one for about 200 balls and one for about 80. What I found interesting was that every one of the balls in the 283 - 283.9 class were from the second session.