I don't think the tiers necessarily correspond directly to "difficulty to achieve." Think more in terms of "level of desirability to the Navy."
Perfect example: Elementary Particle Physics is Tier 2.
In most cases, pure math and science majors tend to be more theoretically challenging than engineering majors, but engineering majors tend to be more practical in an applied technical environment. If you are trying to hire technical problem solvers, you are most likely going to seek engineers and technologists over math/science purists.
As a computer engineer, I find it interesting that I would have been Tier 2. I know I did better than some 4.0 Aero Engineers on aero tests in flight school ... they were so busy arguing the details while the
real engineers were racking up the test scores.