Answer:
The myth is that Supreme Court Justices often surprise the Presidents who pick them.
The reality, according to the article, is that while there have been a few exceptions, most Justices perform as might have been predicted and are only perceived to be more liberal or conservative than expected because of changes in the court's ideological balance over time. The article argues that Presidents can effectively anticipate where prospective nominees stand on the litmus-test issues of our time by looking at their résumés.
The author uses the historical example of Justice Hugo Black to make the point that Justices are often perceived as having "moved to the right" or "moved to the left" when, in fact, they are staying true to their judicial philosophy. President Franklin D. Roosevelt appointed Black to counter the conservative court's use of the due-process clause to strike down parts of the New Deal. Black's more restrictive approach to due process, which was considered liberal at the time, fit Roosevelt's agenda. However, when the Warren Court tried to use the due-process clause for their own purposes, Black balked and was perceived as having "moved to the right." The example illustrates that Justices' views on constitutional interpretation remain consistent, even if the political landscape changes around them.