Only one of these probabilities corresponds to what you'd see if you opened the other two boxes a bunch of times. What sort of actual expectation does the other one control?
My second issue is that "SIA" and "SSA" are not magic wands. They are tools derived from more fundamental considerations that have limited ranges of applicability. And these fundamental considerations are not all that complicated (I'm preparing a post on the topic) - talking about them is a much better and non-opaque way to deal with these problems.
Problem numero tres: This result only holds for big systems if your total expected frequency of life -> 0, not just the density per cell. So doesn't work well for the universe
Consider a scenario in which there are three rooms. In each room there is an independent 1/1000 chance of an agent being created. There is thus a 1/109 probability of there being an agent in every room, a (3*999)/109 probability of there being two agents, and a (3*9992)/109 probability of there being one.
Given that you are one of these agents, the SIA and SSA probabilities of there being n agents are:
The expected numbers of agents is (1(3*9992) + 2(2*3*999) + 3(3*1))/(3*1+2*3*999+1*3*9992) = 1.002 for SIA, and (1(3*9992) + 2(3*999) + 3(1))/(1+3*999+3*9992) ≈ 1.001 for SSA. The high unlikelihood of life means that, given that we are alive, both SIA and SSA probabilities get dominated by worlds with very few agents.
This of course only applies to agents who existence is independent (for instance, separate galactic civilizations). If you're alive, chance are that your parents were also alive at some point too.