I did a project report on dice statistics in college. The part that applies here is that the number of dice rolled can have an increased chance of favorable result. This is why the game of craps only uses two dice.
Example: 1d6 has 3 chances of yielding a 4+ result. 2d6 have 6 chances, however, the physical interaction between dice makes this more like 7 chances. 3 separate rolls of 3d6 would then yield 11 chances each totaling 33 chances. A single 6d6 might also have 33 chances.
So at cruiser or frigate values the difference is negligible but for tier 1s with escorts it could make all the difference in the world.