As far as I know, SP uses the same list of random values for all functions involving randomization. If you need ‘real’ randomness, you can use an expression like:
t = Time.now.to_i
use_random_seed t
puts rand
But actually a list of random values as the base of a randomization is very valuable, because it makes randomness repeatable (which seems to be contradictory): The assumption behind this concept is, that in a musical context you might indeed want to have some sort of randomness but once you share the code it should be the same choices as in the moment you created this randomness in the first place.
So, as far as I understand and appreciate, randomness in Sonic Pi means: Let it choose the first time and make this choice repeatable in order 1. to share and 2. to use it as creative tool.