I actually think there is technically no max on the boards (or if there is, it's ridiculously high). I've been told that board scores are based on the national mean and standard deviations, with your absolute score (number of question right/300) being converted into a released value based on its relationship to the national mean - the goal being a mean national score of 216 with an SD of 24. So technically, if a given year has a more difficult exam (with a lower average absolute score) then the maximum released score goes up. That crazy 280 is probably a reflection of a great score on a tougher exam - nice work!