Had an eeg done to look for seizure activity. no seizures were found, but brain activity between hemispheres was different. what can cause this?

Asymmetric EEG: can be caused by structural lesions ( slowing or lower amplitude) a difference in thickness of the skull on one side, a substance between the electrodes and the cerebral cortex ( like a collection of blood ) . Sometimes the difference is due to a problem with the EEG equipment ( usually this is eliminated as a possibility with calibration procedures. or one can born with asymmetric brain