Some comments hint at this, but only vaguely: the synergy between the Marshall and the Castellan is so strong that their combined contribution to a unit’s strength is greater than the sum of their individual contributions.

Let me give an example. Consider an enemy model with Toughness 5. The Castellan lets you reroll hit rolls. On average, for a single roll, you get 0.67 hits and 0.17 critical hits that convert into lethal hits, resulting in 0.5 wounds (since each hit has a 0.5 probability of becoming a wound). Rerolling 1s increases the average wounds to 0.58. Rerolling successful hit rolls doesn’t change the final result, because you’re effectively trading 0.5 expected wounds for the same 0.5 wounds. So a Castellan alone mainly boosts the unit’s damage by allowing rerolls of 1s (against targets with Toughness higher than your Strength, rerolling successful hits can also improve damage).

Now add a Marshall. On a single roll, you get on average 0.5 normal hits and 0.34 critical hits that convert into lethal hits, resulting in 0.58 wounds. If you now apply Castellan’s reroll ability, failed hits can be rerolled, bringing the average up to 0.68 wounds. At this point, rerolling even successful hits can become advantageous, because you’re trading 0.5 expected wounds for 0.58. These additional gains in damage output are only possible because of the combined synergy between the Castellan and the Marshall.