PCB stackup ambigous percent number
The percentage shown is most likely the thickness left over once the PCB is pressed.

Rogers is a ceramic material, which doesn't shrink however the resin that sticks the layers together does. That also explains why FR4, a woven and resin filled material shrinks more compared to Rogers.

It is strange, however, that the two 1080 prepregs that are in the middle, which have an initial thickness of 75 micron each, and a final thickness of 70 micron each once pressed, have not been specified as percentage. Maybe the shrinkage is already included in the figures listed for the outer layers.