Since Step 1 is a Standardized exam, by my estimations the highest grade is between 290-300. They give you the mean, and standard deviation. To simpify things let's assume a normal distribution.
Last year's mean was a 217 with a SD of 24. This means one standard deviation is 241. Two standard deviation is 265, and three standard deviation is 289. 3 standard deviation is 99.87 percentile on a Z table. Which means top 0.13 of all test takers. If we estimate that 40,000 students take Step 1 each year (from First Aid). This means .0013 x 40,000. This is about 52 people above 289. The number 52 is significant because if 52 different test forms were available the top scorer for each form would have to be given 289 since all of the forms have to be standardized against one another.
I think we can assume that there are multiple forms availble for step 1. When I say multiple forms I mean different content. In the MCAT, all of the forms with the same second letter were the same exam in different order. For example form AN and CN had the same questions but in different order. However forms AN and AB had different questions altogether. Similarly, USMLE Step 1 has similar forms but in different order as well as exams with different content all together.
If we assume SD of 3.3. than we multiply .0005 * (40,000), the answer is 20 people. The limit seems to be 300. Then if there are 20 test forms available it would seem likely that the top scorer of each form would be given the max value somewhere between 290-300. Thus, the maximum score is likely below 300 since the USMLE Step 1 is a standardized exam.
Scores above 3.3 SD above the mean is mathematically unlikely (in a standardized exam) when the sample size is a relatively small finite number of less than 40,000 students i.e taking Step 1 exam.