The US election is an election of elections.

They break the election into smaller elections, based on where people live. It doesn't matter if someone gets 99% of the votes in that area, or 50.0000001% of the votes. Winning is winning.

So if someone wins 49% of the smaller elections, but wins them with a high enough amount, they can have more total votes and still lose the whole thing.

Edit: Let's say there are only 3 areas of 10 people each.

Area 1: Goes to Person A
Person A: 9
Person B: 1

Area 2: Goes to Person B
Person A: 4
Person B: 6

Area 3: Goes to Person B
Person A: 4
Person B: 6

So person B wins 2 of the 3 mini elections, which makes them win the whole thing.
Person B had 13 votes, Person A had 17 votes.
Person A would win if it was "popular vote"