maybe it is qualitative not quantitative!!
Maybe it depends on the quality of questions not the quantity!! I mean let's say step one exam is 2 parts not just one , one for essential knowledge and that is the major part of the exam which extends quantitatively (by the 3 digit score) up to 240 as it includes most of the questions and the other smaller part is related to knowledge beyond the essential and that represents about one sixth of the questions so it determines three digit score beyond 240. this explains why 99 extends from 240 to 280!! it's just an idea...