My guess for what that meant was different: that you could go to the test room any time in that window, but his emphasis "if you have to give birth, you're going to give birth in the exam room" suggested that once you entered, you were under some observational time control and limits on reference materials for the duration of the test. That is, it was not a 'take-home' test.
The bimodal distribution he showed did not have the massively sharp mode, or bulge at perfection, that might be expected if people were looking at the answer key while answering. Instead, it appeared to be two normal distributions, with different averages, superimposed. That suggests to me imperfect pre-test memorization of a larger answer pool, some of which was forgotten during a monitored in-room test.
The bimodal distribution he showed did not have the massively sharp mode, or bulge at perfection, that might be expected if people were looking at the answer key while answering. Instead, it appeared to be two normal distributions, with different averages, superimposed. That suggests to me imperfect pre-test memorization of a larger answer pool, some of which was forgotten during a monitored in-room test.