The reason is that observed inflation rates can be written as steady state value times one plus the percentage deviation:
pi_obs=pi_ss(1+pi_hat)=pi_ss*pi_hat+pi_ss
For example, if pi_SS is 1 and pi_obs=1.1, inflation is ten percent above its steady state and hence pi_hat is 0.1.This is just an example of an observation equation (see the lower part of [Simple model. Unkown error!))
But more importantly, what are you trying to do exactly? What do you mean with disinflation process? Do you simply mean a period where inflation is below its steady state value due to a series of shocks? Or do you mean a structurally different phase where the inflation target was lower than before?