Why are news outlets calling different results in US election?

The US election 2020 is in full swing with the results looming around the corner. The wait will not be long and by most accounts Joe Biden is leading the presidential race, weighing heavily US President Trump’s hopes for re-election.
While Business Insider and Voxi have already called the election for Democratic hopeful, Joe Biden, most outlets have not gone to this extent.
BBC reports Biden’s electoral college votes at 253 while The Guardian is reporting a number of 263.
The various news outlets have been reporting different results pouring in from the states. The rationale behind is that the media outlets go by the majority vote count reached by a state as there does not exist a single governing body for the elections.
These desks at media organisations then either include or exclude the electoral vote in the main tally.
The main news agencies reporting or taking these calls are Associate Press, Fox News, NBC, and CNBC etc.
Here is one example, where Director of the Elections Unit at NBC News explained the win in Pennsylvania
With 88% of the votes counted, Biden is leading with 264 electoral college votes while Trump is at 214, according to The Guardian tally.
Meanwhile, the New York Times has been sticking to 253 votes for Biden and 214 for Trump.
Either of the candidates is to clinch 270 electoral college votes to claim victory in the presidential polls.