How does someone end a game and win with less points and when their opponent still has tiles

How does a game end when your opponent has more tiles to play and is the high scorer? The game ended with the low score a winner?

Words With Friends uses the same endgame rules as Scrabble. The game ends when there are no tiles left to draw and one player has used all their letters (or after 3 passes/swaps in a row).

If the game ends because a player has used all their letters, two things happen:

• The other player (or players, if Zynga ever release a 3+ player version) lose points equal to the value of their hand.
• The player who ended the game gains that many points

This is likely what happened- your opponent ended the game. You lost points and they gained points from your remaining tiles and this was enough to put them in the lead.