Well, taking it out of the abstract, these kids were probably given math problems that were boring by the time you got to the second set. There's only so much 254+739=? you can do before you get bored. At that point, they weren't testing motivation, they were testing patience.
Unfortunately if you go back to the actual research, kids who were complimented for being smart when given the choice preferred to be given easy problems for homework, and the ones who were told they were hard working preferred to be given difficult problems.
I take that as evidence from the students that boredom wasn't the cause of their results.