Google's election results data visualization
Possibly because google was predicting based on changes from previous elections, possibly combined with opinion polls, on a precinct-basis. If all the pro-Trump precincts report in first, you'd still see a blue box because of the very low probability of historically Democrat precincts voting Trump. 

"Calling" something in a US election is not an official result, its a confident prediction by an agency. Sometimes these are wrong. Search for "Dewey Defeats Truman" for a classic example.