The issue is, if you set up the experiment as described, and repeated it millions of times, you'll find that the vast majority of the times that a 7 was chosen, the selection came from the small urn, if you have a 50/50 chance of choosing either urn. Of course, if you have a much greater chance of choosing the bigger urn, say proportional to its size, then you can have the 7 come equally often from either urn. It's all a question of the rules.