The file from midiworld.com has no doubt, as others have mentioned above, been rendered into midi from a human recording. Since humans don't perform digitally, no two notes in the performance are likely to have the same lengths digitally, even though their values may be the same on the printed page. Unfortunately, although different "quantizing" schemes may "fix" the problem to more or less degree, they usually produce odd "stutters" as JIK has mentioned above. I find that, when I enter a piece of music into NoteWorthy by hand and produce a midi from it, that midi can be imported back into NWC with good results - but that's because the original midi was produced from my "digital" entry, not from a performance.
Using Firefox, I was able to view the music on "Take a Sheet Music Break" with no problem, and don't understand the problem you're having with it, leeneia. To me, it looks pretty much like the music in my printed copy of the inventions as I remember it. I wouldn't think a midi copy would be an improvement unless you want to edit the score.