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Because of the need to do it in a way that is both plausible and not readily detectable.

If you go too far down the rabbit hole, you'll find that's what some folks believe. Been there, got the t-shirt. Ultimately not sure where I stand now.

I think the idea is that you want the results to "match-up" with exit polls and number of voters.

I assume it adjusts the proportionality so the sum total remains the same, and ballot questions that the hijacker is not concerned with are not skewed.

Since you have no idea how many people will vote until the conclusion, if you wait until after the conclusion to modify results, it would be more obvious and difficult.