Because the values of sedimentation rates are not additional during ribosome assembly (80S ribosomes correspond to eukaryotic ribosomes).
The rate of sedimentation is calculated by a formula which depends at the same time on the molecular weight, but also on the radius of the molecule (if two molecules are added their molecular weights will be added but not their radii).
The ribosome is a complex composed of RNA and ribosomal proteins, associated with a membrane (in the granular endoplasmic reticulum) or free in the cytoplasm. Common to all cells (prokaryotes and eukaryotes), the ribosome (and especially its composition) varies according to the organisms, even if it is always composed of two distinct subunits.
Prokaryotes have a 70S ribosome (S corresponding to the Svedberg sedimentation unit) composed of 50S and 30S subunits. Three ribosomal RNAs are involved in its structure (23S, 16S, and 5S) as well as 55 proteins.
The eukaryotic ribosome is called 80S, divided into two 60S and 40S subunits. Four RNAs (28S, 18S, 5.8S and 5S) make up its structure with more than 80 proteins.