The US election result is delayed because provisional and mail-in ballots are difficult for election officials to process, according to an expert.
NBC News' director of elections, John Lapinski, says the decision desk is facing issues with provisional and postal votes as many of them will be later "rejected".
Provisional ballots are used by voters if there is a question about their eligibility when they show up at the polls - meaning they take longer to process than regular votes and can be subject to legal action and challenges.
He said the "big issue" is that officials have a rough idea of the number of provisional and mail-in ballots, but they are "hard ballots for election officials to process because most of them have problems."
"And so we're going to see a lot of rejections, so that's why there's this uncertainty," he added.
Typically, provisional ballots make up a tiny fraction of the vote - just 1.8% of votes cast in the 2016 presidential election and around 70 of which were accepted, according to the US Election Assistance Commission.
However, voting has been significantly different this year because of the large use of mail-in ballots due to the COVID-19 pandemic.
Mr Lapinski said despite President Donald Trump netting ballots in the likes of Pennsylvania and the provisionals while his rival Joe Biden nets the remaining mail-in ballots, "we don't have a really good sense yet of how these provisionals will be rejected, and how many of these mail-in ballots will be rejected".
The large number of provisional ballots in Pennsylvania "wasn't that much of a surprise because of a lot of the changes that have happened in this COVID era and how elections are administered," Mr Lapinski said.
"But what did surprise us is that provisionals usually break Democrat, and we are not seeing that at all in Pennsylvania."
He said that while the decision desk is "always cautious", this particular year has thrown "so many curveballs our way", so they are taking more time to understand the data.
"It is taking us a bit more time, we are giving it a second and third set of eyes to make sure we are seeing the same thing," he said.
Talking about how news networks decide to call a winner, Mr Lapinski said: "We need to make sure that we are 99.5% confident, and that the person who is leading isn't going to be caught by the second candidate."
NBC News is working with Sky News on the election results.