I'm not sure what you mean by "statements" in Prolog as it's not a term the language defines. If you're referring to clauses, it's not true that execution is unordered: the Prolog interpreter attempts to unify a goal with clauses from the knowledge base in the order they appear. This ordering is semantically significant for control flow.
If instead you're referring to goals within the body of a clause, this is also incorrect. Goals are evaluated strictly left-to-right, and each must succeed before the next is attempted. This evaluation order is likewise required and observable, especially in the presence of side effects.
If instead you're referring to goals within the body of a clause, this is also incorrect. Goals are evaluated strictly left-to-right, and each must succeed before the next is attempted. This evaluation order is likewise required and observable, especially in the presence of side effects.