When I’ve voted I was told that if I made a mistake, for instance, I would need to turn in my ballot that I had marked wrongly so they could void that number - thus explaining why the results would be less one ballot. They’d give me a new ballot, they said. The point being that they had to account for what happened to all the ballots they handed out. If somebody crumpled up their whole ballot I would imagine that an election worker would have to void that ballot number, because that ballot was not used to submit a vote.
But to fill out one page and then go to the trouble of treating the other page differently in order to not submit it, rather than simply putting both sheets together into the sleeve seems like more trouble than a frustrated voter would want to go to. If you’re in a hurry, vote for what you care about, skim over the rest to see that you don’t care, and then stick it all in the sleeve and head out.
But the tabulation of undervotes tells me that they kept had to have a way to know which page 1 goes with which page 2 - which suggests some kind of serial/ballot number that matched page 1 to page 2. And if that was the case, it makes no sense for the software designers (literate people being paid to do a specific, detail-oriented job) to use the number of pages cast to calculate the “% Turnout” on their summary report.
For my three-card ballot I cast on Tuesday, here's how each card was numbered (per the strip torn off each card I got to keep as a sort of receipt):
Side One: A-Card 1, [number1] [(number2)] [number3] BT:3 (right side)
[number4] - POLL, [number5] [barcode square graphic]
Side Two: [number4] - POLL, [number5] [barcode square graphic]
Side One: B-Card 2, [number1] [(number2}] [number3] BT:3 (right side)
[number6] - POLL, [number7] [barcode square graphic]
Side Two: [number6] - POLL, [number7] [barcode square graphic]
Side One: C-Card 3, [number1] [(number2}] [number3] BT:3 (right side)
[number8] - POLL, [number9] [barcode square graphic]
Side Two: [number8] - POLL, [number9] [barcode square graphic]
There were three numbers consistent between the three ballots, and two which were unique to the ballot. I believe the "BT:3" indicated there were three cards per ballot.