here's a reductio ad absurdum example. party A and party B, support for whom is split precisely across the electorate.
in an election with a 70% turnout, the result could swing from a 50:20 vote for party A vs party B, to a 20:50 vote for party A vs party B, without a single eligible voter changing allegiance.
in an election with a 70% turnout, the result could swing from a 50:20 vote for party A vs party B, to a 20:50 vote for party A vs party B, without a single eligible voter changing allegiance.