If a person is an American, then he is probably not a member of Congress.

This person is a member of Congress.

Therefore, he is probably not an American.

If a person is an American, then he is probably not a Russian.

This person is a Russian.

Therefore, he is probably not an American.

Both of these have false statements in the third position. The problematic word is 'therefore'. Most Russians aren't Americans, but that's not because most Americans aren't Russian; it's because most people don't have dual citizenship (among other possible facts that you could infer that from).