Sometimes, it’s the scale. Hundreds of thousands of votes take longer to tally than just a few, so huge urban areas often lag behind smaller places. Other times, it’s the mail. California, for instance, where there are seven tight House races, is notoriously slow, in part because more than half of voters opt to use vote-by-mail ballots (a.k.a. “absentee” ballots in some places). California ballots postmarked on Election Day have three days to show up at county elections offices. A few other states allow a week or 10 days; Alaska will accept ballots from abroad up to 15 days later. “I’ve always speculated about a worst-case scenario where an Alaska Senate seat could determine control of the U.S. Senate, and there may still be ballots sitting at local ‘post offices,’” said Paul Gronke, director of the Early Voting Information Center at Reed College, in an email. “Post office,” he said, could actually mean a remote bait shop or grocery store from which ballots would need to be airlifted, validated and counted.
Gronke said some jurisdictions that depend heavily on mail-in voting get quicker results by processing the ballots as they arrive rather than saving all the envelope-opening, signature-validating, scanning and such until after the polls close. That way, they just need to hit the button to count the ballots.
Traffic could be a factor, too. A few jurisdictions do “central counting,” where all ballots are delivered to a single location after the polls close to be counted all at once. One of those is massive Los Angeles County, where some votes arrive by helicopter and boat.