8-layer Stackup: 3 cores vs 4 cores
This is due to how PCBs are built. Inner layers are etched in pairs to copper that is pre-laminated to both sides of a core. Outer layers are bare copper sheets that are etched when the whole PCB is already laminated. Thus 8 layer boards almost always have 3 cores and 4 prepreg layers because there are three pairs of inner layers. You can find videos in youtube etc that explain how multilayer PCBs are manufactured.