If you download a file and your media player shows a different length than the preview did, this is an issue with your media player. If you actually watch the media play and time it, you will note that the length is the same as the preview, not the time that your media player is showing.
This is a common issue with Windows 7 / Windows Media Player. If you download the media on to another computer, you should also be able to verify that the length matches with the preview. If this is the case, then the solution would be to update your operating system.