How fit is calculated
The checker compares resolved millimetre dimensions with the selected usable build volume after subtracting your margin. Automatic mode tests only axis-aligned, right-angle permutations; it does not claim arbitrary-angle optimisation.
Why a slicer can still reject a model
Nominal build volume can differ from the usable region. Multi-colour purge towers, support space, sequential-print clearance and manufacturer exclusions can require more room than a bounding box.