If ionly did 2yrs why must i do 3years on post release controland not aloud to get off early?

my last case thou i did 2yrs and got off prc in one yr ..this time they say im not aloud to get off earley ....idid 2yrs in prison then did 2 more yrs 2 years later i did both of my senteces day for day..idont owe restiution or fines ....they say im not aloud to get off early but why must this be if a person does histime why is prc longer than the sentence??????