I think that is possibly well above my payscale to answer that one. I have long given up trying to fathom the itineries that el taskers come up with.
My only guess is that the C17 had freight which could not be taken on the Tri*, and was needed to sweep. Often the Tri* used on trails are K1 which have no cargo door, therefore the sweeping herc or C17(?) will carry the spares and freight. Just a guess.