Depending on where you look, Joe Biden is projected to have won 253 or 264 or 273 electoral votes. Why the discrepancy? There are several independent organizations making race calls. Each has a 'decision desk', where experts (statisticians, political scientists) analyze incoming election results, and mathematically model what's yet to be counted. When they are highly certain that the final numbers for a race will favor one candidate, they will make a call. For example, NBC and ABC use a 99.5% level of certainty before making a projection.
As each of these decision desks works independently, and are sequestered from outside influences, races will be called at different times, although absent the rare situation where a call has to be retracted, they will all eventually get to the same place.
Here's how things stand as of Saturday morning, courtesy of this excellent interactive from The New York Times. In the graphic below, we are showing the states/districts that remain uncalled by one or more outlets.