Though the effect was stronger in those with lower intake, the effects on mood in those with higher intake were still quite substantial, (full text). That is to say, both groups benefited. Selenium improved anxiety only in the low intake group, (full text).
Interestingly, the high and low intake groups had the same baseline scores. That is to say, it's not that selenium brought the low intake group up to normal, but rather that they were lifted above the high intake group. It's possible that they had adapted to their low intake, be it psychological or physiological adaptation. I recall a similar effect with creatine and cognitive performance in vegetarians.
This raises the question: does the benefit disappear over time as one adapts to their new selenium levels? Perhaps, but I find it more likely that the benefit drops only slightly. That is, I think what may be occurring is a a positive feedback loop where better mood makes you more optimistic, thus improving your mood; I expect this psychological mechanism to fade, leaving the biological component intact.
Of course, there is the possibility that this is a statistical fluke. Nonetheless, I'd expect the above mechanism to occur in general. If I learn more about statistics I could probably run a p-value test.