Can someone explain the way Period_work_days works in custom reports

Using Help the definition indicates "The number of working days in the current period taking into account the resource working and holiday calendars."

When I list a number of resources, I wish to obtain the period workdays it only appears to work for the excavator mining the pits and not the loaders I am using to transfer ore from stockpiles. In my case we are using an ore haulage contractor so a loader to feed them and a loader to transfer from various stockpiles to feed the plant. Each of these resources has their own calendar consisting of different holidays (for periods not worked). I am not sure what I am doing wrong, or if it only works for resources mining the pits.