Scheduling question

I’ve always heard that if a store doesn’t use all of its budgeted hours, those hours can be lost rather than carried over to the next month.

My question is: If a store has full-time employees who are consistently being scheduled below their expected hours (for example, 32–35 hours instead of 40), why wouldn’t the store manager use available hours to give those full-time employees more hours before giving additional hours to part-time employees?

Is there a Walgreens policy or labor-budget rule that prevents SMs from doing this? Or is it mainly up to the SM how they distribute the available hours?
Just trying to understand how the scheduling/labor budget actually works at Walgreens.