On election night we get a number, a graphic and a speech. Weeks later the official result comes out, and it is not always the same. Both are real - they are just counting different piles.
Election night feels like the end. Somebody makes a speech, somebody else makes a sadder speech, and the graphics settle down.
Then, weeks later, the official result is declared - and sometimes a seat has changed hands.
The reason is that election night is a preliminary count. It is fast on purpose, because a country wants to know. It counts the ordinary votes cast in the electorate people are enrolled in, and it does it under time pressure on the same evening.
What it does not include is special votes. A special vote is what you cast when the ordinary way does not fit: you are enrolled somewhere else, you are overseas, you enrolled late, you voted from hospital. Those votes take longer because each one has to be checked against the roll before it can be counted at all.
That checking is not a delay caused by someone being slow. It is the check that makes the vote count legitimate.
Special votes are a big share of the total, and they do not always lean the same way as the ordinary votes. So the official count can shift a party's share slightly, and in a close electorate a slight shift is the whole race. Under MMP, a small change in party vote share can also move a list seat from one party to another.
So there are two true statements that look like a contradiction. On election night we knew the result. Weeks later, the result was different. Both were the best answer available at the time, and only one of them was ever called official.
Question for the reader
Imagine you get to choose. Option one: a fast result on the night, knowing it might change. Option two: no result at all for three weeks, and then one number that never changes.
Which would you pick, and why? Think about who benefits from the fast version, and who is harmed if people treat a preliminary number as final. What should a news bulletin be required to say when it puts a result on screen?