I don't know the answer, so I'm purely speculating for the fun of it.

I guess that hash function outputs aren't perfectly uniformly distributed. E.g. if a toy hash function (a) produces a 2-bit output for a gajillion random inputs, you wouldn't get a quarter of the values in each bucket. Maybe you'd get 30% in 00, 20% in 01, and 25% in both buckets 10 and 11. Salting the inputs wouldn't help with that. It'd only make similar inputs less likely to collide, but collisions would still be more likely in the worst case.

By combining different a hash function (b) with different "lumps", I suppose that the lumps would even out so that you'd approach a probability of .25 in each bucket.

        00   01   10   11
  a    .30  .20  .25  .25
  b    .22  .22  .25  .31  
  a+b  .26  .21  .25  .28

Therefore, if you're going to spend time hashing something more than once, you might as well use different hash functions for each cycle.