If the movies you were downloading were MPEGs, I can see two different files merging fine (though there'd be no guarantee that the different segments break at the same spot in the movie or that you're not concatenating two completely different movies). However, if the movies are AVI's, I don't think it would work. AVI's are a lot pickier about corrupt data / changes in format.

So, either you were downloading MPEGs, or....

There's a bug in the segment comparison routine that will cause it to fail the check even when they two segments overlap properly. I find it hard to believe that there are many instances where people have put up files of the exact same video, that ended up being the exact same size, that happen to be at the exact same point in the movie at the segment boundries, that are so indistinguishable that you don't see a major glitch at that point during it's viewing, that are so unique that there are differences in 2 tiny kilobytes where they overlap (1/367001600th of the file for a 700MB movie).