I have doubts about the offered explanation.

From the abstract of the study behind the article [1]:

    When people seek to impress others, they often do so by
    highlighting individual achievements. Despite the intuitive
    appeal of this strategy, we demonstrate that people often
    prefer potential rather than achievement when evaluating
    others. Indeed, compared with references to achievement
    (e.g., “this person has won an award for his work”),
    references to potential (e.g., “this person could win an
    award for his work”) appear to stimulate greater interest
    and processing, which can translate into more favorable
    reactions. This tendency creates a phenomenon whereby the
    potential to be good at something can be preferred over
    actually being good at that very same thing. We document
    this preference for potential in laboratory and field
    experiments, using targets ranging from athletes to
    comedians to graduate school applicants and measures
    ranging from salary allocations to online ad clicks to
    admission decisions.

What causes this apparent preference for potential achievement over actual achievement? According to the study's abstract (the study itself is behind a pay wall), it's a mind trick: uncertainty "stimulates greater interest and processing, which can translate into more favorable reactions." More of the same from the HBR piece:

    When human brains come across uncertainty, they tend to pay
    attention to information more because they want to figure
    it out, which leads to longer and more in-depth
    processing. High-potential candidates make us think harder
    than proven ones do. So long as the information available
    about the high-potential candidate is favorable, all this
    extra processing can lead (unconsciously) to an overall
    more positive view of the candidate (or company). (That
    part about the information available being favorable is
    important. In another study, when the candidate was
    described as having great potential, but there was little
    evidence to back that up, people liked him far less than
    the proven achiever.)

That's a nice theory, but another explanation, which doesn't seem to have been considered, is that in many cases something that has the potential to reach a value of X is actually more valuable than something that we know, with certainty, has a value of X. That's because a thing that has the potential to reach X also has the potential to reach beyond X, but a thing that has already been measured to be X does not.

In other words, you are given two probability distributions: one wide, one tight. The wide one, you are told, has the potential to contain X. But the tight one, for certain, is centered on X. Now, which do you pick?

If you're in a situation were values of X are nice but values beyond X are gold, then you pick the wide distribution. That's because the tight distribution is centered on "nice" and, being tight, offers almost no hope of straying into "gold" territory.

So maybe it's not a mind trick, after all.

[1] http://psycnet.apa.org/psycinfo/2012-18069-001/

EDIT: fix typos.