The stackup is made of a central core. then above and below prepeg layers are added I think. I think it is the way of manufacturing PCBs. Technology Manufacturing related.
Prepreg, which is an abbreviation for pre impregnated, is a fibre weave impregnated with a resin bonding agent. It is used to stick the core layers together. The core layers being FR4 with copper traces. The layer stack is pressed together at temperature to the required board finish thickness. Prepreg comes in different thicknesses.