In the interest of giving credit where credit is due, my avatar is a scan of a hand-drawn caricature by cartoonist Jim Naylor which was done at my company's 20th anniversary dinner.
3 players play Catan (the base game), all have 9 points, the person who get 10 points first win the game.
Player A has the longest street. Player B, rolled the dices and builds a new town in between the longest street. Now A lost the longest street title to C and has only 7 points left, B got his 10th point and C has the longest street now and 11 points.
Who won the game? B or C?
B - because it was their turn. You can only win by getting 10 (or more) points during your turn.
Catan Rules section headed Ending The Game (p5) wrote:
If you have 10 or more victory points during your turn the game ends and you are the winner! If you reach 10 points when it is not your turn, the game continues until any player (including you) has 10 points on his turn.(The bold text is also bold in the rule book.)