I didn't downvote you, but I think they object to your overly literal interpretation of "one ball is heavier than the others" given it was contrasted with the original phrasing "one ball is a different weight". This is a math puzzle, not a lateral thinking puzzle.
Not to draw this out further but I do think variations of these make for good interview questions for entry-level people. Here is why:
First, it is trivial to communicate the problem and any moderately competent person will find a solution within a short time frame. Second, it tests how precisely someone takes a problem definition. (In my experience, the kind of person who glances over such details will also tend to make trivial programming mistakes like off-by-ones.) Third, by varying a well-known question slightly you can easily filter out those who have simply learned the "correct" solution from the internet rather than thinking it through.