So if you, standing somewhere in line, suddenly decided not to vote, the votes from people ahead of you would already be cast; but those from people behind you would be left up in the air.
In this model, if you vote for a winning candidate, the chance that your vote was the marginally-pivotal one that puts your candidate over the top is still exactly 1/N where N is the number of voters for that candidate; but the ex ante utility of committing not to break the chain is far higher.