Hmm...well I would believe that you're playing the wrong way and perhaps are holding down the strings on the frets in the wrong place or in the wrong way?...hope that helped...
While not being an expert at setup by any means I think fretbuzz is usually a result of faulty techique. It just takes practice and learning how hard to press down the string. Also, try pressing down just next to the fret (as in not between the frets). Actually I think right hand touch also can affect fretbuzz.....
On the other hand your bass could be setup wrongly...
