Different videos can have different runtimes, and those various runtimes are shown in "owned videos" and "existing videos".
For the movie itself, we provide the official runtime. In this case, 51 minutes isn't a mistake.
As you can see on the IMDb, in the section Technical Specs: Runtime
http://www.imdb.com/title/tt0010323/this movie is "51 min (video) | 73 min (DVD)".