of two.

Essentially, any bits above the next lowest power of two are less likely
to be set.  For example, xp_random(6) would use 3 bits of randomness with
the following distribution:

000 = 0
001 = 1
010 = 2
011 = 3
100 = 4
101 = 5
110 = 0
111 = 1

As you can see, zero and one are twice as likely to occur than 2,3,4, or 5.