My 11 months nephew had seizures. his eeg test came out normal, ct scan result reports he has mild cerebral and cerebellar atrophy, how it happened?

About EEG, seizure: If there is atrophy (means smaller than normal) please see a pediatric neurologist. Eeg's can be normal in the context of seizures for many reasons. If they are abnormal, they are often helpful, but treating your nephew clinically is most important, and eeg's are a first step to diagnosis with prolonged studies or other types of eeg's as a next step.