Good call on checking with a gauge and not just relying on the OEM dash gauge.
Without a secondary gauge and relying on the factory gauge, there's no way to know how much of a drop is actually occurring
Most likely problem is just general wear on bearing surfaces leading to out of spec clearances on things like crank main and big end bearings and crank journals surfaces, and camshaft bearings and cam journal surfaces.
As bearings wear, clearances increase, your oil pump is pumping the same...