This is becuase although you were able to figure out the problem, based on the time that it took you, you only scored 60 % of what was available, nearly a draw, statistically speaking. The amount of time it takes you to finish a puzzle is heavily factored into your score. You can read more about the ratings and the Tactics Trainer here: http://www.chess.com/tactics/help.html.
trainer rating issue
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sorry if there is a post about this already, i did not see one
I'm looking at my tactics training score, which has gone down despite not failing a puzzle. I had a 1268 rating, completed puzzle 21811 at 60% + 30 but ended up with a rating of 1244 (a minus 24 points instead).
I'm just pointing this out if it's a bug or to see if i'm missing something in the scoring.