Calculus of Consent -some questions.

If anyone is reading this I just have one question: What does "Uncertainty" mean in this book? (If you're not reading it and want to help, I think econlib has it online.) Anyway, on page 78, or the last section of the "Economic Theory of Constitutions" called "Some Qualifications" the authors state: "essential to the analysis is the presumption that the individual is uncertain as to what his precise role will be in any one of the whole chain of later collective choices that will actually have to be made. For this reason...he cannot predict with any degree of certainty whether he is more likely to be in a winning or a losing coalition on any specific issue. Therefore he will...choose rules that will maximize the utility of an individual in a series of collective decisions with his own preferences on the separate issues being more or less randomly distributed." If they are saying that individuals will choose constitutional rules that will bring greater "happiness" to any arbitrary person, that prefers the same thing as they do and who vary their preferences randomly due to uncertainty; then why would they assume at the chapter's beginning that, "the specific institutional structure through which collective action is to be carried out is exogenously fixed" and that "the choice among the...[constitutional] rules is not independent of the choice as to the method of organization [which I take, to mean the same as "specific institutional structure]." For if this is what they really meant, then wouldn't the choice of constitutions, be already dependent on the institutional structure? and if that is true, then wouldn't the rational individual predict a certain set of outcomes based on this institutional structure and not any random outcome; that is, wouldn't uncertainty be non-existent? Note: in the book the constitution is different from the institutional structure.