I have seen two different uses of "Who's to say X". It appears to me, that the author could either defend X in a sarcastic or ironic way, or could attack X by presenting evidence contrary to X. My question is: is my understanding correct in that "Who's to say X" could be used in two opposite ways?
Example 1: Who’s to say that we’re to blame for global warming? Source 1
In my understanding, example 1 means: I challenge you to prove we're to blame for global warming. We're probably not to blame for global warming. That is, there are external factors or circumstances outside our control which cause or at least contribute to global warming.
Example 2: Medicare is portrayed as getting the best deal from the system because Medicare pays less per service. But remember how the system works. Who's to say Medicare doesn't pay less per procedure because it's being billed for many more procedures, because that's how providers are allowed to maximize their revenues from the payer known as Medicare? In fact, plenty of evidence suggests this is exactly how Medicare operates. Source 2
In my understanding, example 2 means: Medicare is commonly portrayed as less costly. I challenge you to prove it costs more according to this complex and not frequently-analyzed approach. Well, in fact, it does cost more! And exactly due to the aforementioned, not well-known technique!
Is my understanding of the usage a correct one? Particularly, in example 2, I feel that my interpretation could be improved.
Thanks for any help.