SS Total = SS Within + SS Between
SS Total is the sum of all squared deviations of individual observations from the overall process mean.
SS Within is the sum of all squared deviations of individual observations from their subgroup means.
Thus, SS Within is purely a function of the inherent variation of the process. However, SS Between is a function of the natural variation in subgroup means and changes in subgroup means that are caused by shifts and drifts in the process mean.
SS Between is the sum of squared deviations of subgroup means from the overall process mean.