A change to the forecast for an existing schedule would require a reschedule to pull in the differences. Historical shrinkage does take into account your environments efficiencies and inefficiencies to a degree and will affect staffing requirement.
Without knowing much, I would say the biggest issue is how low the staffing requirement is. Is this a BU where the volume is very low? In those case even a small change in volume, AHT, or even just the arrival pattern can seem like big changes on the staffing requirement. If your load and staffing are overly 'fractured' into very small numbers you lose the economies of scale and get into the realm of 'small number problem'.
If nothing obvious, I would recommend opening a support ticket. My article below also notes several things you can investigate: https://help.mypurecloud.com/faqs/why-are-staffing-requirements-higher-or-lower-than-expected/
------------------------------
Jay Langsford
VP, R&D
------------------------------