Yes but you don't know all of the variables and they can sometimes change. For example if your an obese alcoholic smoker then you will die at 45. If you stop eating, drinking fluids, and breathing you will die right now. So of course it can change. Suddenly x has changed values! It's no longer representative of 4 it's representative of 5 years left of your life where x is that number! Oh no, your life is going from the variable number to another one! Suddenly it's gone from x_1 to x_2! What do? So therefore it's not mapped out for you :)

That's the point, if you theoretically knew all the variables in the universe, and not just the macro ones, the atomic change and placement of each individual bit of matter. Because they too, influence the universe. These variables would be able to predict the change in other variables and so on, therefore there would be an extremely complex tree of variables from the beginning of time and all those factors would eventually lead up to the chemicals in ones brain telling them to drop dead. In this theory about the universe, there is no probability, there is only variables so small and unmeasurable to us, that we write them off as pure chance.