The 80S ribosome is identified according to its sedimentation coefficients in Svedberg units.  Since it is a measure of sedimentation time, it is not simply a sum of subunits. Svedberg units are a nonlinear function, and 60S and 40S are two separate subunits that together sediment at 80S. If it were a matter of size or mass, they could be added up to 100S, but the Svedberg unit is a measure of time based on speed instead.