For the US presidential elections, the Associated Press announced winners for several states although a significant portion of the votes were uncounted.
One example is Illinois where at the moment of writing (12:32 Central European Time) 80% of the votes are counted
- Mr. Biden received 55,1% or 2.881.554 votes
- Mr. Trump received 43% or 2.246.472 votes
that sums up to 5.346.276 votes. If 80% of the votes are counted that would make that the total amount of votes is 5.346.276/0.8=6.682.845.
That means that 6.682.845 - 5.346.276 = 1.336.569 votes are uncounted. Which means that theoretically Mr. Trump could still get 2.246.472 + 1.336.569 = 3.583.041 votes. And thus could still win the state, although Mr. Biden was already announced the winner.
Although this is statistically unlikely it would not be impossible. And this kind of bothers me, because I took an extreme example but one could imagine less extreme cases where the results could really change.
It would seem reasonable that only if the total number of remaining/uncounted votes is less than the difference one could announce a winner, but apparently they use much more flexible criteria. What are the limits of probability applied when prematurely announcing winners before all votes are counted?
Note that this question is not about the particular example, but about the general principle of officially announcing a winner while theoretically the other candidate could still win.
EDIT based on the given answers the following part is not correct.
Also one could consider possibilities of "legal" "fraud" where some of the votes, would just be counted later. Say During manual counting 10% of votes for Candidate A are put aside for later counting, or votes from particular regions with particular preferences are counted later. In such scenarios without really changing any vote from A to B election results could still be influenced just by postponing a selection of the votes.
EDIT My previous understanding was that the states were called before all votes were counted and that they would not continue to count the remaining votes. But based on the answers this information was only a projection so all votes will still be counted, and there is no fraud except for the influence that preliminary results might have on the voting behavior in states where voting was still open, due to the time difference.