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It's pretty simple if management can't 'manage' hours correctly.
Not required for duty means you stay home with zero deducted hours (hence the shortfall come end-of-year) - additional work means there is overtime to be had (i.e working rest days).
Like said above, that's the way the roster falls sometimes. If you have AV days (ie not required) then you have the day off but no hours deducted from your contract. If they call you in with less then 24hrs notice you can either refuse duty or the hours worked will be added to your rest day count.
I'm on target for a similar situation this year though with not quite so many rd hours.