After all, while psychometricians definitely have above-average IQs, they probably don't have IQs over, say, 160.
If psychometricians already know what the right answers are (and how to obtain the right answer), then wouldn't they score very high on the tests, even if they're not THAT smart? (making it possible for other people to score high on the tests if they studied psychometrics as well?) The format of the test isn't going to vary that much from administration to administration. Of course, the tests are timed, but it's entirely possible that the practice the psychometricians get from writing the tests could easily make them better at each of the subsets of an IQ test.