why postgresql define NTUP_PER_BUCKET as 10, not other numbers smaller

When I read the source code about the hashjoin, I was very confused that the
postgresql define the NTUP_PER_BUCKET value as 10.
Since this value is used to estimate the tuple count in one bucket, is it
better if we have a smaller value?
I have not done some experiments, but it seems that we could archive less
hash collisions and better performance if we decrease the value.
So could anyone explain to me that why we define NTUP_PER_BUCKET as 10?
If there exists a specified situation that we would get worse performance or
some troubles if set NTUP_PER_BUCKET to 1 or 2?

Thanks very much. 



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