The counting of votes in contemporary American elections is usually not completed on Election Night. There has been an increasing tendency for vote shares to shift toward Democratic candidates after Election Day in general elections. In this paper, we study this phenomenon using granular data from Orange County, California. Leveraging snapshots of precinct-level election returns and precinct-level demographic and political composition, we conduct the first full-fledged analysis of the potential drivers of vote share shifts. Utilizing snapshots of individual-level administrative records, we provide the first analysis of the characteristics of voters whose ballots were tallied later versus earlier. Far from being anomalous, the vote share shifts are consistent with underlying precinct voter compositions and the order of individual ballot processing. We find the same underlying drivers in North Carolina and Colorado and discuss the implications of the evolving election administration practices across states for public concerns about election integrity.