Emergent phenomena are like that. You can track them down, but not up.
For example, consider a complicated piece of software. Sometimes it seems to act on its own, or hate you, or something. I swear that one Linux distro I tried had a thing against me personally. And this behavior is usually unintended by the programmers. Often it even appears non-deterministic and hard to duplicate. However, one can usually trace the weird behavior to a bunch of "bugs", rather than attribute agency/free will to a piece of code. But it is nearly impossible to predict the weirdness beforehand. Stuff just happens. Apparently even the term "bug" came about because any non-trivial piece of technology appeared to have gremlins inside just trying to mess with you.
Given that humans are probably just glorified computers evolved out of meat, it seems unsurprising that we have developed a mind of our own.
I think that I have two possible errors in my argument:
I agree with you that behavior in complex systems seems non-deterministic even though the underlying processes are deterministic (like the software example you give above).
So our perception of consciousness and even the dichotomy "randomness vs certainty" could be a mental illusion.
I was trying to reconcile the fact that in a deterministic universe there could be life with free will, but I am going full circle now and am starting to think that everything is really random, if not I don't see how there could be free will in a deterministic universe.
If mathematicians measure randomness with probability, then there must be some things that have a 100% occurrence probability (in the current universe above atomic levels I presume), which now I see as special cases of randomness rather than opposites to randomness, and these lead us to think that there is determinism.
I think we may have this cognitive bias (deterministic views of reality) because it is extremely helpful to use these 100% probability occurrence things to model the universe rationally, learn, and to predict the future, but it is not the whole story or at least a complete description of reality.
What do you think?
EDIT 1: Thank you all for the comments below. I recognize I am naive in this topic.
Although I am not convinced yet, I think my possible argumentative error is:
P1: I observe free will in the behavior of living things.
P2: Deterministic physical mechanical processes don't permit free will.
C: Therefore physics must include random processes.
I think I only see a solution of free will in randomness, but maybe there are other solutions ( I will read the Free Will Sequence here on LW!)
EDIT 2: After reading some articles of the Free Will Sequence I realize the problem of investing energy around free will questions if free will is just a mistake in our thinking process.
It is something like why ask about time travel if time doesn't exist? or, why explore the mechanics of randomness vs determinism if randomness doesn't exist and thus the dichotomy "randomness vs determinism" doesn't exist in the first place?