Quoting from here:

The numbers x and y are part of the experiment. How they came to be is not part of the question asked, and therefore questions about how to choose them do not enter into the problem.

Despite tilly's meticulous attempts to befuddle, bemuddle and obfuscate, there is an implicit assumption made. The numbers have a defined relationship, a comparison function. In the absence of further restraints, they form an open-ended (infinite) ordered set.

Drawing elements from a finite set, randomly and without bias, yields n**3 equally likely hands. In this abstract case, the set is infinite. Elements are drawn from the same set because there is an (implicit) mutual understanding, and presumption of rationality.