Why are actual core losses larger than calculated?

When calculating the core losses, it is assumed that the structure is homogeneous. In reality, when core halves are mated, there is leakage flux (fringing flux) at the mating surfaces, and the gap losses contribute to the total losses. Gap losses are caused by flux concentration in the core and eddy currents generated in the windings. When a core is gapped, this gap loss can drastically increase overall losses. Additionally, because the cross-sectional area of many core geometries is not uniform, local “hot spots” can develop at points of minimum cross section. This creates localized areas of increased flux density, resulting in higher losses at those points.