Coaster capacity on quiet days could be todo with staffing levels, as more staff are needed to run extra train on most costers , either so that in the event of a breakdown there is a member of staff with each train and to man entrance ect or because the ride setup needs more staff eg stealth
Extra staff used to start later in day for break cover ect.
it doesn't excuse poor operations or capacity but merlin ultimately give thorpe staff hour budgets to work to.