Just goto the edit movie window and select 'change movie', this will let you select the correct movie manually.
As for the why, it is one of those little issues that makes it so difficult to scrape from IMDB with 100% accuracy, it would ber easy to fix it so that this movie was scraped correctly, but it would stop many more movies being scraped correctly.
I have an idea that will improve the situation, but it's on the backburner on the moment, i'm in the middle some serious rewriting.