Youtube Activity for Safari

Ok well, I'm pretty sure some of you know that you can go to a youtube video, click on windows > Activity and then double click the link to make it a file. Well when i convert the file into a mp3, a regular video that should be 4 mins long turns out to be 27 mins long. Lets say its a song, so when I listen to the song normaly, it will say the song is 27 mins long but when i listen to it without fastforwarding or anything, the song ends when its suppost to but if i fastworward it, the song will go past the 4 mins and onto 27 mins...its very hard to explain but if anyone knows what im talking about, any help! Thanks :S